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Journal of the Biological Society
Victoria University of Wellington
New Zealand
Parasitic plants are those which grow on other living plants, and derive part or the whole of their food supply from their hosts by organic union with them. They should be distinguished from Epiphytes which grow upon other plants but have no organic union with them, e.g. Asplenium flaccidum, the fish-tail fern, is an epiphyte which often grows in the accumulated humus of a tree fork but does not develop any contact with the living tissues of the tree. The parasitic method of nutrition is made possible by special structures called haustoria, by means of which the parasite is closely attached to and penetrates the host tissue. These haustoria invade the vascular system of the host, and divert food materials and water from the host xylem and phloem into the parasite. The parasitic plants described here are all obligate, i.e. they can live only parasitically, and may or may not be deleterious to their hosts. They can be classified either (a) as Holoparasites which have little or no chlorophyll and are wholly dependent on their hosts, or (b) as Hemiparasites which possess chlorophyll and are capable of photosynthesis, but depend on their hosts for water and mineral salts.
The parasitic habit is found in widely different families of the Dicotyledons and in one family of Monocotyledons, so there can be little doubt that it has evolved separately in these various lines of descent. Altogether about 1,400 species of parasitic Angiosperms are known, occurring chiefly in the following groups: Archichlamydeae, Santalales, Loranthaceae (the Mistletoes): 30 genera, 520 species, common in tropics; the New Zealand genera are Loranthus, Elytranthe, Tupeia and Korthalsella, all with endemic species. (The little-known Phrygilanthus possibly also occurs in New Zealand, but is not described in this account.) Balanophoraceae: 15 genera, 40 species, tuberous root parasites, tropical. The New Zealand representative is the monotypic Dactylanthus taylori. Santalacea: 26 genera, 250 species, mostly tropical root parasites, no parasitic forms known from New Zealand. Aristolochiales, Rafflesiaceae: 7 genera, 22 species, tropical root parasites, none from New Zealand. Hydnoraceae: 2 genera, 7 species, South American and African root parasites, none from New Zealand. Ranales, Lauraceae: Cassytha is a parasitic herb member of this tropical and subtropical family of trees and shrubs. New Zealand has one species
Sympetalae, Tubiflorae, Orobanchaceae: 12 genera, 140 species, cosmopolitan root parasites. One species of Orobranche has been naturalised in New Zealand. The Scrophulariaceae includes a tribe, the Rhinantheae, with 11 genera and 350 species of root parasites. The Convolvulaceae includes 10 species of the parasitic Cuscuta. Several of these have been introduced into New Zealand, though there is possibly a distinct native species. There are also a number of monocotyledonous root parasites belonging to the family Orchidaceae, the New Zealand representatives of which belong in the genus Gastrodium, but are not dealt with here.
The New Zealand parasitic flora includes at least 10 genera, from six different families. Some of these families are exclusively parasitic, e.g. Loranthaceae; in others there are some genera with the ordinary autotrophic method of nutrition and some with parasitic methods, e.g. Lauraceae. The percentage of parasites in the total flora is very small, but as a group the parasites are extremely interesting because of their morphological and physiological adaptations.
The following key can be used to identify the main New Zealand plant parasites to the generic level. The noteworthy features of each of these genera are then discussed, together with a brief account of the species they include. For full descriptions of the species, reference should be made to Cheeseman's ‘Manual of the New Zealand Flora’.
The Dodders are parasitic flowering plants of the family Convolvulaceae. The seed germinates in the ground and sends up a reddish or purplish, filamentous, leafless stem, the free end of which, in the course of its circling growth movement, may contact a suitable host. The thin wire-like stem then entwines the host, developing minute haustoria or sucker pads on the side of the stem in contact with the host plant. The haustoria arise from the internal tissues of the parasite and penetrate the vascular tissues of the host, absorbing nutriment directly from the phloem and xylem and diverting it into the tissues of the parasite. At first the peg-like processes consists of undifferentiated cells but later those adjacent to the xylem become tracheids and those in contact with the phloem develop phloem-like elements. Once the parasite is thus attached, its seedling-root dies. The Dodder is then entirely dependent on its host for its water and mineral salt supply, and, since it has little or no chlorophyll and cannot synthesise its own food, manufactured organic substances must be derived, pirate fashion.
There are several species of Dodder which have been introduced into New Zealand, and Cheeseman records one native species, Cuscuta densiflora, which is closely relately to, if not identical with, those introduced. The introduction of these parasitic plants has serious economic implications because they attack and may eventually kill such leguminous crop plants as lucerne and clover. The Dodders deplete their food supply and impoverish the crop plants sufficiently to cause death. The thread-like stems are produced so rapidly that the host plant often appears to be smothered in a mass of intertwining threads on which fascicles of flowers may develop later in great profusion. Sometimes the parasite develops branches which attach by suckers to adjacent host plants. If left unchecked the pest can thus spread through and destroy an entire crop. The Glover Dodder, C. epithymum var. trifolii, is the most destructive species and has been designated ‘Devil's guts’.
This is one of the most curious of our native parasitic plants being a root parasite of certain trees and shrubs in scattered localities in the North Island forests. It belongs to the Balanophoraceae, a family of tropical root parasites, and is known only from New Zealand. The plant consists of a brown, rounded, tuberous rhizome 1-12 cm. in diameter, covered with hard, wart-like tubercles, and it is affixed to the modified termination of the host root. The plant is mostly subterranean, the brown, scaly inflorescences, 2-6 in. high, being the only portions projecting above the leaf mould on the forest floor. The flowers are unisexual and closely packed together in heads surrounded by overlapping brown scales. They have a sweet, heavy perfume which in many cases has led to their detection.
In the autumn the terminal scales of the inflorescence bud open to expose the capitulum of spadices. It is presumed that pollination is brought about by insects which are attracted by the flowers' scent.
In New Zealand homes the so-called ‘Wooden Roses’ of Dactylanthus have become collectors' pieces. It was the common practice of bushmen when they discovered the rhizomes of the plant to cut off the parasitised host root, break off any flowering stems, and then boil the host root with its terminal parasite in water until the whole of the parasite together with the bark of the root could be removed. The modified terminal portion of the host root is then seen to be hollowed and fluted and to resemble a finely chiselled ‘wooden rose’. (Fig. D.)
Dactylanthus is a complete parasite, lacking chlorophyll and deriving its food supply entirely from the host plant root system. The specialised layer of cells at the junction of parasite and host acts in an haustorial fashion. These cells are contiguous to the phloem and xylem of the host and derive food material for the parasite. Dactylanthus has no true haustorial organs; it is also interesting in being relatively long-lived and having stout and bulky tissues. For a detailed account of the anatomy and reproduction, see Moore (1940).
This is a leafless twining parasite similar in habit to Cuscuta and, like it, deriving most of its food from the host through penetrating cushion-like haustoria in contact with the xylem of the host. It grows on living shrubs and trees, and has pale, yellow-green, wiry stems which are several feet long and much interwoven. Very small scales replace the leaves. It is a common parasite on Manuka hosts in the gumlands of the far north of New Zealand, where it smothers the trees with its yellowish wiry stems. Unlike Dodder, Cassytha may have a small amount of chlorophyll, and is therefore capable of synthesising a little of its own food.
Cassytha occupies an isolated position in the Lauraceae, a family of otherwise woody plants, including Tawa and Tairaire. The only New Zealand species, C. paniculata, also occurs in Australia.
This is a common introduced parasite which attacks the roots of native and cultivated plants and weeds. It sends up an erect, purplish brown, succulent
New Zealand has three species of Pygmy Mistletoes which were first put into the Mistletoe genus Viscum by Hooker. Now, however, the three closely allied species are placed in the separate genus Korthalsella. They are small, tufted, succulent plants, 2-5 in. high, leafless and with conspicuously jointed greenish stems, and belong to the family Loranthaceae.
The jointed stems are terete in K. salicornioides (Fig. H) and closely resemble the round succulent stems of the glasswort (Salicornia) of tidal salt-marshes, hence the specific name. This species is found as a parasite on Manuka. In K. Lindsayi (Figs. I and J) and K. clavata (Fig. G) the internodes are flattened. In the former the internodes are broadly obovate and in the latter they are more attenuated.
The greenish flowers are minute, borne with one male and four female flowers grouped together, two groups forming a whorl at a node. The seeds are small and sticky. Germination of the seeds may take place on a suitable host or sometimes on the parent plant, resulting in a form of cannibalism. Attachment to the host is achieved by the seedling haustorium penetrating the host stem and spreading out in a club-shaped head in contact with the vascular supply of the host stele. Carbohydrates, water and mineral salts are derived from the host by the parasite which, however, is able to supply some of its own manufactured food since it possesses some chlorophyll.
Details of the morphology and reproduction of the New Zealand species can be obtained from a paper by Stevenson, 1934. Recorded host plants for K. salicornioides include Leptospermum spp., Gaultheria, Dracophyllum; for K. Lindsayi, Sophora, Melicope, Metrosideros, Myrtus, Coprosma, Suttonia, Helichrysum; and for K. clavata, Aristotelia fruticosa, Discaria, Coprosma. These parasites occur throughout the length of New Zealand but only rarely and locally.
This is an endemic parasitic shrub with thin, narrow, pale-green leaves, oval to oblong lanceolate in shape, which are usually arranged in opposite pairs on pale grey stems. It is a member of the family Loranthaceae. Its yellowish green flowers are dioecious, or occasionally hermaphrodite, and are arranged in small axillary and terminal bundles. The seeds are ovoid and sticky, and are white or pinkish. The attachment to the host is by an haustorium at a single point. This parasite is found on a variety of hosts, although the most usual is the endemic monotypic Carpodetus serratus (Putaputaweta).
The life history of the plant is described by Smart, 1952.
The New Zealand representative of this large tropical genus of parasitic shrubs (of the family Loranthaceae) is characterised by having small greenish flowers whereas other Loranthus species often have highly coloured flowers. It parasitizes Coprosma, Melicope, Leptospermum, and also introduced Acacia, Rhododendron, Poplar, Plum, Apple, Hawthorn. It contains chlorophyll in its tissues and is able to manufacture its own food to a limited degree, but is dependent on the host for water and mineral salts. Its yellow viscid berries are dispersed by birds. The seeds are left on branches where they germinate, each giving rise to a long root-like organ which follows the course of the host branch, attaching itself into the host tissues at intervals by suckers. The young foliage is bronze coloured at first, but later becomes green.
Loranthus micranthus, the common New Zealand
The seedling development and haustorial development is described in Menzies (1954).
Cheeseman describes four endemic species of parasitic green shrubs belonging to this genus, which is a member of the Loranthaceae. The plants parasitize Nothofagus, forming large bushes in the tops of the trees. Elytranthe colensoi, which bears abundant racemes of bright-scarlet flowers 1 ½-2 in. long, can make a spectacular display in the flowering season amidst the dull foliage of Nothofagus. It can grow on other hosts. E. tetrapetala (Fig. F), the Pirirangi, also bears red flowers, but they are produced singly or in small groups. It grows on Nothofagus and also Quintinia in the North Island. A yellow-orange flowered species, E. flavida, grows on native beech from East Cape southwards. A less common species with duller red flowers, E. adamsii, grows on Great Barrier Island and the Coromandel area. Its hosts are Coprosma, Suttonia and Melicope.
Each year usually brings to the writer of this article at least one student who, wishing to prepare a solution containing picric acid, is surprised and perturbed to find the stock bottle of this substance containing‘… a lot of watery liquid above the crystals’. Students are even aggrieved, thinking that some previous user has been careless, and are annoyed with the presumed prospect of having to dry out the wet mass of crystals.
Since there appears to be a steady stream of similar misconceptions, the following notes might profitably be drawn to the attention of all biologists, especially those engaged in histological work.
Picric acid (2 : 4 : 6 trinitro-phenol) is a common ingredient of fixatives and even staining solutions used in histological and cytological techniques in biological laboratories. Its use depends on several important properties including a strong precipitating action on cytoplasmic proteins without concomitant hardening (though it does promote shrinking); and penetration of chitin. It is worth while noting, however, that there are stringent precautions to be observed in handling this material which is better known in the field of explosives. ‘Picric acid if stored in bulk should, for safety, first be damped. Smaller quantities may be safely kept whilst dry, but should be stored in bottles having cork or rubber stoppers; glass stoppers should never be used for potentially explosive substances, because on replacing the stopper some of the material may be ground between the neck of the flask and the stopper, and so caused to explode.’ (Mann and Saunders.) The necessity for ensuring that non-rigid stoppers are used and that the material be maintained damp cannot be over-emphasised. Fortunately precautions can be readily observed in biological laboratories, since wide-mouthed containers with large corks can be employed for storage, and the concentrations of picric acid required are low — they can easily be derived starting from the saturated aqueous solution from the stock bottle. Thus, it is recommended that a stock bottle (say, 1 litre capacity), suitably stoppered with cork or rubber, be maintained with a layer of picric acid some 3 cm. thick and filled with distilled water. The picric acid dissolves slowly to form a saturated solution of which the concentration varies with temperature as follows (data from Seidell):
Recipes for preparing fixatives incorporating picric acid are very conveniently given in terms of a dozen or so ‘basal fixatives’ of which volumetric quantities are combined variously to give a great range of standard fixatives. (For details see Gray or Bolles Lee.) Consequently, a saturated solution of picric acid being one of the basal solutions, it is never necessary to contemplate weighing out dry picric acid. Where picric acid in alcoholic solution is called for, this may conveniently be prepared by filtering the water-wet crysals, preferably through a Büchner funnel under suction, washing with alcohol and taking up the crystals from the filter paper in alcohol of the appropriate strength. One of the basal fixatives is 95% alcohol saturated with picric acid; this may therefore be obtained by allowing the acid to stand covered with 95% alcohol, and the supernatant solution withdrawn as required, replacement being made each time with 95% alcohol. Picric acid is several times more soluble in ethyl alcohol than in water, maximal solubility being achieved at about 80% alcohol.
Particular attention is to be drawn to the picrates. ‘Metallic salts of picric acid are much less stable than the free acid, and should always be stored damp.’ (Mann and Saunders.) Fortunately, very few fixatives in common use include both picric acid and cations of heavy metals (which produce the most dangerous picrates), but those which do should be handled with respect in the realisation that the metal picrates are even more explosive than is the acid itself. One of the standard tests for explosiveness is that of the ‘fall-hammer’ in which a 2-kilogram hammer is allowed to drop on to the substance under test. The following figures, taken from International Critical Tables, indicate the potential dangers involved: height of fall necessary to explode picric acid — 35-95 cm.; ammonium and sodium picrates — 80 cm.; zinc picrate — 60 cm.; copper, iron, lead and
Emphasis is laid on the fact that heavy metals are common impurities of other chemicals, especially where commercial grade chemicals are being used; traces of such metals, inadvertently and unconsciously introduced into picric acid solutions are hazardous. The cautions against allowing the solution to evaporate to dryness and against the use of glass stoppers are therefore strongly reiterated. They should be applied to all stock solutions and working solutions of picric acid, picrates, fixatives and staining solutions containing any of these materials, whether in aqueous or alcoholic solution.
The birds considered here are all, with the exception of the giant petrel, members of the family Diomedeidae, the albatrosses. Although variable in size they are all large birds with long narrow wings and a distinctive energy-conserving method of flight.
The petrel order, Procellariiformes, to which the albatross family belongs, is, apart from a combination of less striking features, characterised by the structure of the bill. This is covered by a number of distinct horny plates instead of a single sheath on each mandible as in other orders of birds, and the nostrils open on the upper bill surface at the ends of short tubes. A dead albatross on a beach may always be recognised by the large bill with a prominent terminal hook, the characteristic covering of plates mentioned above, and by the nostril tubes which are separated by the upper bill plate. At sea, their large size, long narrow wings and characteristic gliding flight are distinctive. The giant petrel is the only other oceanic bird which may be mistaken for an albatross, and although it belongs to another family of petrels, it is included here to avoid confusion with the Sooty Albatrosses.
The distribution of albatrosses is interesting. Of the thirteen species of albatross currently recognised, three belong to the North Pacific, one is equatorial but ranges south into temperate waters, while the remaining nine species with some four races or sub-species, are confined to the southern ocean between 30° and 60° south latitude. Apart from occasional southern species, probably carried north by ships, albatrosses do not now inhabit the North Atlantic, although fossil bones of Diomedea have been reported from Europe in rocks of Lower Miocene age.
Most ornithologists divide the thirteen southern forms into three groups, each containing birds of similar appearance and obvious relationship. It is this arrangement which is followed here. The first group to be considered contains the largest flying birds, the Wandering and Royal Albatrosses (genus Diomedea). Both are of about equal size and easily recognised by their white backs and tails. It is this group for which the
The second group, which also belongs to the genus Diomedea, includes those albatrosses generally referred to as Mollymawks. These are smaller than the great albatrosses and easily distinguished from them by the dark back, wings and tail (see figs. 1 and 2), and the usually more colourful bill.
The Sooty Albatrosses (Phoebetria) make up the third group. These are somewhat smaller than the Mollymawks and uniformly dark in colouration. They have dark bills, exceptionally long wings, and a long pointed tail, this latter in marked contrast to the short square tail of the Great Albatrosses and Mollymawks. Of the two species of Sooty Albatrosses, one is not positively known from New Zealand waters. The other, while not regularly observed at sea except in more southerly latitudes, is sometimes washed ashore on our west coast beaches as far north as the Auckland Peninsula.
While the style of flight is not uniform in the order Procellariiformes, it is similar in the larger members. These agree in having long wings with numerous short secondary feathers. Such a long narrow wing gives a maximum efficiency in gliding flight, which, while fundamentally characteristic of the whole order, is seen most clearly in the albatrosses. The wings are held horizontal or downwards, not upwards as in the soaring flight of eagles and vultures, and are frequently flexed as if adjusting balance. The velocity of the wind appears to determine the speed and ease of flight. A minimum wind velocity of about 10 m.p.h. seems necessary to sustain the type of flight practised by an albatross. Given these minimum requirements the bird rises at a steep angle against the wind, swings across wind with one wing pointing downwards, the other upwards, before finally making a rapid descent downwind and repeating the circular movement. From the water, albatrosses achieve flight by running across the surface with stiffly beating wings until airborne. As a rule the actual launching takes place from the crest of a wave after a short run of only a few yards. However, under light wind conditions, or during a calm, flight may not be attained in under 100 yards, and when achieved is usually laboured with a heavy wing flap, the birds soon settling again on the water, where they are frequently observed with the bill tucked into the scapular feathers and apparently asleep. High wind velocities of 40-50 m.p.h. induce faster, more buoyant flight, accompanied by more rocking in the apparently increased air turbulence. Under these conditions,
Most albatrosses inhabiting the southern ocean are ashore, and commence breeding, in spring and early summer months. The Royal Albatrosses, the species most intensively studied and to which the following notes refer, is ashore on its breeding territory in October. The nest is a low mound of soil, peat, and other debris, with a hollow crown on top roughly lined with grass. A single, large whitish egg is produced during late November or early December. Incubation commences immediately, the chick hatching some seventy-nine days later, about the middle of February. For the first five weeks of life, the chick is guarded by one or other of the parents, but thereafter, except for feeding which continues until it flies, it is left to its own devices.
The fluffy down has disappeared by the thirty-fourth week and the fully-fledged juvenile departs between the thirty-sixth and thirty-eighth
Immature Mollymawks are not treated here at any great length, as all are difficult to determine either in flight or on the water alongside a vessel. Usually first-year birds have a black bill and a darker head and neck than the adult. Underwing pattern is generally similar to the adult although during the first few months of flight may be partially obscured by a dark flush. Of the great albatrosses, immature ‘Wanderers’ are a rich brown with the face white and a largely white under-wing, while immature ‘Royals’ resemble the adult.
Albatrosses feed chifly on squids and fishes, but also eat other marine animals (including small sea birds) and galley refuse from ships. Fishing vessels in our coastal waters are frequently attended by numbers of albatrosses scrambling for discarded offal.
While the specific status of most albatrosses breeding in the New Zealand region is well understood there still appears some doubt as to the identity of the races of Black-browed Mollymawk breeding on Macquarie Island.
The following key is designed to facilitate identification of adults in flight, or settled alongside a ship. The reference numbers, e.g. 1 (6), are alternatives. Where there is not agreement with 1, refer to 6. Where the agreement is with 6, then proceed to the next number, i.e. 7, etc. The illustrations show colour pattern only, not relative size or necessarily proportion. For details of life-history, etc., the reader should consult the references following the key.
The writer is indebted to Dr.
Professor of Botany (on sabbatical leave), University of California, Los Angeles.
It is now nearly one hundred years since Julius Sachs advanced the idea of specific organ-forming substances in plants, and
This article is a summary of knowledge of the more important hormones and growth factors of plants. The need for brevity has led to omission of many interesting observations, details of which are in the references. Definitions. Order is slowly emerging from the confusing duplication and overlapping of terminology in the field of plant regulators, aided by the increasing knowledge of their chemical nature. The following definitions and/or descriptions of terms are based in part on the report of a special committee on nomenclature of the American Society of Plant Physiologists (Overbeek, et al).
Plant Regulator — a chemical which in small amounts regulates (promotes, inhibits or otherwise modifies) a physiological process. A broad term which includes the hormones (endogenous regulators) as well as exogenous regulators.
Plant Hormone — an endogenous regulator. To be a hormone, a chemical must be produced within the plant, transported from a site of production to a site of action, and be active in small amounts.
Vitamin — an organic chemical required in small amounts from external sources for the normal growth and maintenance of an organism. Green plants (autotrophic organisms) do not require vitamins. Non-green plants and animals (heterotrophic organisms) usually require one or more vitamins.
Auxin — The generic name for the indole hormones. These promote growth by stimulating cell enlargement. A number of other effects of auxin are described in the text. (Indoleacetic acid was called heteroauxin in the older literature. The hypothetical auxin a and auxin b have never been isolated and are now generally considered invalid.)
Antiauxin — (synonyms: auxin inhibitor, auxin competitor, auxin antagonist). A compound which competitively inhibits (in the biochemical sense) the action of auxin.
Inhibitor — a regulator which inhibits or retards a physiological process. Growth Substance — a term used variously, usually as a synonym for auxin. Growth Factor — usually an exogenous regulator, often equivalent to vitamin or hormone. Frequently used with reference to the culture of micro-organisms, tissues, or organs.
Growth Hormones: Auxin. Darwin's experiments led him to conclude that something is produced in tips of plants which moves downward and stimulates the growth of more basal tissues. In the early part of this century a number of investigators studied the nature of the influence of tips on the growth of basal regions. This work culminated in the simple but conclusive experiments of Went which showed that the growth of the oat coleoptile requires a chemical which is produced in the tip of the coleoptile. The chemical was called auxin. It is widely distributed in plants and appears to be universally required for growth, hence Went's Law of plant growth: ‘Without auxin, no growth.’
Indoleacetic acid is now considered to be the principal auxin, but there are several other auxins. All are closely related chemically, and more than one is often present in a tissue. The best known of these are indoleacetaldehyde, indoleacetonitrile, and ethyl indoleacetate.
Auxin has several physiological properties which account for most of its functions. It promotes cell enlargement by increasing the plasticity of the cell wall, thus facilitating the uptake of water. It is transported basipetally (from tip toward base) in organs. This transport is modified by gravity and light: auxin tends to move in the direction of the force of gravity, and away from a source of light. Further, auxin in the plant is inactivated by light. Also, the different organs of plants differ in their sensitivity to auxin; roots are the most sensitive, stems are the least sensitive. Through the interaction of these properties, growth and growth movements of plants are regulated. (The growth movements include phototropism, geotropism (both positive and negative), plagiotropism, thigmotropism, epinasty, and photonasty.)
Auxin has other functions. Its accumulation at the base of a cutting is necessary for the initiation of roots. Auxin from the apical bud inhibits the development of lateral buds. The auxin gradient across a leaf or fruit abscission zone regulates abscission (a relatively high gradient retards abscission, and a low gradient accelerates abscission). These three functions at first glance would appear unrelated, but they may all be expressions of another postulated property of auxin, that is, to mobilise food to regions where auxin is in high concentration. The existence of this property has not yet been established, but there is considerable supporting evidence. If it does exist, it would account for much of what is known concerning the auxin regulation of root inhibition, bud inhibition and abscission. Another function of auxin is the initiation of cell divisions in the cambium. In deciduous trees the cambium is inactive during the winter, in the spring auxin moves down from the expanding buds and initiates cell divisions
Continued research on auxin has made it apparent that auxin physiology is much more complicated than it first seemed. Auxin appears to be present in all living parts of the plant, mature as well as immature. The amounts present are effected by at least three general processes: auxin production, auxin transport, and auxin inactivation. Many of the early investigations did not recognise the existence of these three processes and their results must be re-evaluated. For example, many studies of auxin transport did not take into account the probability of considerable auxin inactivation during the course of transport. Auxin is produced principally in young tissues, but can also be produced by mature tissues. The amino acid tryptophan, a common constituent of proteins, is the precursor of auxin, but the precise chemical steps of its conversion to auxin are not yet settled. The transport of auxin can be through the parenchyma, as it is in the oat coleoptile, but in more mature tissues transport is largely in the phloem. In the coleoptile transport is correlated with the streaming of protoplasm. Auxin inactivation is accomplished by an oxidative enzyme which can function either in the dark or under the influence of light. Mature tissues have relatively high auxin-inactivating capacities. In addition to these general processes other factors, still obscure, also influence the auxin in tissues. The interaction of these processes and factors determines the level of auxin which is available to influence growth and morphogenesis.
Growth Hormones: Gibberellins. The gibberellins produce effects on growth, particularly cell elongation, which are very similar to the effects of auxin, but they function in situations where auxin does not promote elongation. Although physiological and biochemical knowledge of them is still fragmentary, they are growth factors which are probably hormones and hence should be included here. The chemicals derive their name from the fungus Gibberella, from which they can be obtained. Immature seeds are also very rich sources.
One of the most interesting series of experiments with the gibberellins was conducted with a dwarf corn (maize). This particular mutant dwarf had been the subject of an intensive auxin study, and its auxin physiology was found to be completely normal. That is, auxin production, transport and inactivation were identical with those of normal corn, and applications of additional auxin did not affect its growth; the plants never grew more than a few inches tall. However, weekly sprays of gibberellins stimulated the mutant to the normal rate of growth and practically normal appearance. The results of a similar experiment conducted several years earlier, which were at the time puzzling, can now be interpreted as due to gibberellins: an extract from immature bean seeds was applied to a bush variety of beans (Phaseolus); the stems then elongated in the manner characteristic of the tall varieties of beans. In other experiments, gibberellins sprayed on pasture grasses have induced abnormally rapid growth.
Another effect of gibberellins is in relation to both growth and flowering. Hyocyamus is one of the typical ‘long-day plants’. It grows as a rosette with its leaves clustered about the very short stem until it has been exposed to a period of cold followed by a period of long days. Then the stem rapidly elongates and produces flowers. It has been found that gibberellins can replace the cold treatment; sprays followed by long days stimulate stem elongation with flowering.
Wound Hormone. Following an injury to a plant, the parenchyma cells underlying the injured area are stimulated to divide and form a protective callus. Under the stimulus, cells divide which would otherwise remain intact to the death of the plant. Early experiments showed that if the injured area is washed immediately, cell division is prevented; this suggested that a hormone might be involved. Such a hormone was isolated by Bonner and English. Starting with 100 pounds of string beans they isolated a small amount of a chemical which they called traumatic acid (chemically, decene dicarboxylic acid) which is the wound hormone of beans. However, this compound does not stimulate cell division in other species. So there remain other chemicals yet to be identified as wound hormones.
Root Growth Hormones. Knowledge of root growth hormones has come largely from experiments with the culture of isolated roots. The repeated attempts to culture isolated tissues of plants were successful in 1933 with tomato roots and a culture medium consisting of sucrose, salts, and yeast extract. Yeast extract is a very complex mixture of chemicals and attention was immediately given to determination of the active components. These were soon found to be thiamin and pyridoxin which in small amounts (a few parts per million) could completely replace the yeast extract. Thus tomato roots, which in the field would live only a few months, have been kept growing in culture in a synthetic medium since soon after 1933. Thiamin and pyridoxin were first called growth factors, since their role in the intact plant was not known. However, Bonner showed that they are produced in leaves and transported downward to roots, thus establishing them as hormones.
Other experiments showed that niacin is a root growth factor, and is presumably also a root growth hormone. In various combinations thiamin, pyridoxin or niacin will support the indefinite growth of isolated roots of many species. For a few species other factors are required such as the amino acids glycine, lysine and arginine.
Although the roots of many plants will grow rapidly (at rates at least equal to the rates of roots on intact plants) and indefinitely in synthetic culture media, important problems still remain unsolved. One is the culture of isolated roots of monocotyledonous plants. In spite of numerous attempts, these have never been established in culture. Another is the development of the cambium, which has not been induced in roots of established
Experiments with root cultures brought to light an important interrelationship of vitamins and hormones. The chemicals thiamin, pyridoxin and niacin are vitamins, necessary in the diet of animals and other heterotrophs for normal growth and maintenance. In the green plant these same chemicals function in the physiological role of hormones. And within the cells of organisms they each function as a part of a vital enzyme. Thus the same chemical may function in any of three physiological roles: vitamin, hormone, enzyme.
Leaf Growth Hormone: Phyllocaline. In a search for hormones other than auxin Went performed an extensive series of grafting experiments. He worked with varieties of garden peas which differed markedly in their growth habits. The results showed, for example, that leaves of different varities differed in their ability to stimulate root growth. Similar differences among roots and buds were observed. Went postulated that these differences in growth were the result of differences in production of special hormones by the varieties. One of these postulated hormones was called phyllocaline. It is produced in cotyledons and mature leaves, and stimulates the growth of young leaves. This hormone was isolated and identified as adenine. Another property of adenine was later discovered; tissue cultures of plant callus ordinarily grow indefinitely as an undifferentiated, or at best, slightly differentiated mass of cells. In the culture medium adenine stimulates the differentiation of leafy buds.
Adenine too has multiple physiological roles: It is a vitamin (B8)
for some organisms and within cells functions as a part of several enzymes and of the energy-storing phosphate compounds.
Flowering Hormone: Florigen. Flowering is influenced by many factors including mineral and carbohydrate nutrition, temperature, photoperiod, and a postulated hormone, florigen. This hormone is produced in leaves (under particular conditions) and is transported to buds where it brings about the conversion of a vegetative stem apex to a reproductive stem apex (flower bud). Numerous experiments indicate its existence, but attempts to isolate florigen have not yet been successful. For further discussion of flowering see the recent article by Sussex.
Reproductive Hormones. In the lower plants a number of hormones influencing reproductive processes have been described, as well as nutritional factors which can be called reproductive vitamins.
One of the best known examples of reproductive hormones is in a heterothallic species of a water mould, Achlya, where Raper in extensive experiments found four hormones:
The function of hormones in one or another phase of reproduction has been demonstrated for:
The following reproductive vitamins have been identified; they must be provided with culture medium for the indicated developments:
Growth Factors. Experiments have demonstrated growth factor requirements for many plant parts. Many, possibly all, of these growth factors are plant hormones, but present knowledge is too fragmentary in most cases to permit positive statements.
Pollen germination and tube growth factors. Pollen of some species will germinate and grow well in artificial media; pollen of others will grow poorly or not at all. Stigmatic exudates are usually very stimulatory and presumably provide hormones required by the pollen. Chemicals which have been found to promote germination or tube growth of various species include: boric acid, manganous sulphate, ascorbic acid, aminobenzoic
Growth factors of tissue and organ cultures. Since the successful establishment of root cultures, other organs and several types of tissues have been successfully cultured including embryos, shoots, and callus. Often successful culture has required the use of complex mixtures such as malt extract, young seed extracts, or coconut milk. The latter is a potent source of important growth factors; its use has enabled the culture of very small embryos, but the active chemicals in coconut milk have not been identified. Growth factors which have been identified include: ascorbic acid, adenine, biotin, indoleacetic acid, niacin, pantothenic acid, thiamin. It is of interest to note that each of these is already known to have functions as a vitamin and/or hormone.
Growth Inhibiting Hormones. The discussion to this point has dealt with hormones and other factors which in the main promote growth and development. (A few of these, such as auxin, will under some conditions inhibit or retard growth.) In addition, there is now an increasing list of chemicals whose principal function appears to be the inhibition of growth. Since these chemicals are endogenous, often act at very low concentrations, and move from a site of production to a site of action, they should be considered hormones. Only seed germination inhibitors will be mentioned here; knowledge of others is very fragmentary.
Germination inhibitors act variously: (a) to prevent premature seed germination; (b) to extend the period of germination by permitting only a fraction of the seeds to germinate at any one time; and (c) to suppress germination of competing species while permitting germination of a favoured species. Evenari has described over 120 inhibitors; these are produced in fruit pulp, fruit coats, endosperm, seed coats, embryos, leaves, bulbs, and roots. Identified inhibitors include: hydrocyanic acid, ammonia, ethylene, mustard oils, aldehydes, alkaloids, essential oils, lactones, organic acids. It is of interest that an inhibitor can sometimes stimulate germination. Inhibition or stimulation may result from different concentrations, but sometimes one follows the other from the same concentration.
In a few decades the subject of plant hormones has expanded to a broad and amazingly complex field of plant physiology, at least equal in complexity to the field of animal hormones. This research received much of its initial impetus from Sachs' postulate that plant morphogenesus is regulated by specific organ-forming chemicals. Indeed, there is now much evidence on the effects of specific chemicals (or groups of chemicals). However, the impression should not remain that morphogenesis is regulated solely by such chemicals (that is, by hormones or vitamins). Temperature, light, water, mineral nutrients, foods, and other factors are also important in the development of plants and at times one or more of these factors may have a decisive influence on growth, acting either directly or through intermediate effects on plant hormones.
For further reading: the Annual Reviews of Plant Physiology contain articles listing and discussing much of the literature, and forthcoming volumes of the Encyclopedia of Plant Physiology will contain numerous valuable articles. The following are selected references:-
Heteropods are marine free-swimming gastropods, and relatively common members of the plankton in tropical and subtropical seas, particularly in the Indo-Pacific region. Occasional specimens may be stranded on the shore, otherwise they are found only in plankton tows. The body is adapted to pelagic life, the familiar creeping gastropod foot being modified as a more or less single, fin-like swimming organ, unlike the Pteropoda where the foot is developed as two anterior symmetrical lobes. The spiral shell may be present, well-formed, reduced, or even absent. Species of the F. Atlantidae are the least adapted to pelagic life, having a spiralled shell containing the visceral mass like most other gastropods, and a flattened, elongate swimming organ developed from the middle and posterior portions of the foot. The F. Atlantidae are also the only heteropods in which the body can be completely withdrawn into the shell, and the posterior region of the foot may carry an operculum. The shell is flattened and rather discoid with the outer whorl showing a pronounced keel. In the F. Carinariidae the shell is greatly reduced, helmet-shaped, and covers only the stalked visceral hump, or a portion of it, about the mid-region of the trunk. ‘Head’ and ‘tail’ regions are well formed (Fig. 1, A), much larger than in the F. Atlantidae and cannot be withdrawn into the shell. Lastly, there is the F. Pterotracheidae in which the shell is entirely lacking, and the small, compact visceral mass is situated in the posterior third of a more or less cylindrical, streamlined body (Fig. 1, F). Species of this family are the best adapted to pelagic life. In both the Carinariidae and Pterotracheidae the foot is relatively small compared with the body, very much compressed and hatchet-shaped.
A particularly conspicuous feature of species of the F. Pterotracheidae is the prominent telescopic eyes, and anyone who is familiar with Zahl's excellent photograph of a living Pterotracheae coronata from the Mediterranean (Nat. Geog. Mag. 1953, Vol. CIV (5)), will agree with his name for this animal — ‘the mollusc with the tear drop eyes’. Unfortunately the eyes in preserved specimens are usually contracted back into the head.
Heteropods are carnivorous and known to be voracious predators of jellyfish, comb jellies, salps, fish larvae, copepods, etc.
The foot is uppermost when a heteropod is swimming, and this is another of their characteristics which makes its difficult to realise that they are closely allied to the creeping common shore periwinkle Littorina. The figures in the present paper show the foot facing downwards because this enables ready comparison with illustrations of other members of the Gastropoda in the literature.
Over the period of a few years, five small heteropods, representing two species, have been found among collections of coelenterates sent to the author. The transparency of the greater part of the body and often also the shell, displays clearly the internal structure, and led to further interest in the group and so to the identification of the above material as well as a larger specimen in the teaching collection of the Zoology Department, Victoria University of Wellington. As descriptions and illustrations of New Zealand heteropods are not readily available, the species in the present collection are illustrated in some detail. They show clearly the characters of the F. Carinariidae and F. Pterotracheidae. Only species of these families are at present recorded from New Zealand coastal waters. A key is also given to assist in recognition of all the known New Zealand species.
Benham's (1905) account of a single damaged specimen of Pterotrachea coronata Forskal washed ashore at Long Beach, Otago, was the only description of a heteropod taken round our coasts until Tesch recorded five species, four of them new to these waters, nearly forty-five years later, in 1949. There are also, however, records of heteropods from open ocean waters, and Carinaria lamarcki was described by Quoy and Gaimard under the name of C. australis as early as 1832 from a locality in the Tasman Sea about halfway between Bass Strait and Cook Strait. Tesch remarks that Firoloida desmaresti ‘swarm (s) everywhere in the Tasman Sea, but disappears suddenly at the South Island of New Zealand (like all Heteropods even Atlanta)’.
Carinaria lamarcki and Firoloida desmaresti are the only two species in my collection. F. desmaresti was taken in plankton tows off Auckland and in Cook Strait; C. lamarcki was taken in Wellington Harbour. As noted above, heteropods live in warm water and it seems likely that a search through other New Zealand subtropical plankton collections and further netting in appropriate localities will show that heteropods are among the common surface water animals of our coasts. Subtropical water may occur seasonally in coastal regions as far south as Otago and the Chatham Islands (Fleming, 1944) and Benham's record of Pterotrachea coronata from Long Beach, Otago, is good evidence for this opinion.
In the present collection Carinaria lamarcki is represented only by a very young specimen 5.0 mm. in length, and Firoloida desmaresti by four specimens 5.0 mm. to 10.0 mm. in length taken off Auckland, and a much larger one 75.0 mm. in length from Cook Strait. In specimens of F. desmaresti 10.0 mm. in length and longer, all the major body regions
F. desmaresti (Fig. 1, C) but the flexure is not evident in the larger 75.0 mm. specimen (Fig. 1, F).
The shell is missing in my specimen of C. lamarcki but the outline of the shell keel can be traced in the dorsal region of the soft tissues of the visceral hump (Fig. 1, A, ke). So far I have been unable to obtain a shell of this species, and Fig. 1, B, shows the shell re-drawn from Tesch (1949).
All the specimens in the present collections appear to be females, the penis being absent from the right side in the specimen of C. lamarcki, while in the specimens of F. desmaresti there are no tentacles before the eyes and no sucker on the foot. These are female characters. Some specimens do not have the egg-carrying caudal peduncle.
As recognised at present, the g. Firoloida is monotypic with F. desmaresti Lesueur, 1817, the only known species (Fig. 1, C-F). Males of this species have tentacles in front of the eyes and a sucker on the foot. Both these structures are absent in the female.)