Tuatara: Volume 4, Issue 2, December 1951
Living Fossils of The Plant Kingdom — The Psilotales — Tmesipteris and Psilotum
Living Fossils of The Plant Kingdom
The Psilotales — Tmesipteris and Psilotum
The psilotales have often been called ‘living fossils’. In some respects, the order Psilotales, with its two genera of primitive vascular plants, Tmesipteris and Psilotum, can be said to occupy a similar position in the Plant Kingdom to that which the tuatara occupies in the Animal Kingdom. New Zealand biologists are fortunate in having these unique plants growing in this country.
Tmesipteris is a small green plant which usually grows from the trunks of tree-ferns, but occasionally grows in humus on logs, or on the ground. In the epiphytic form the fleshy branching rhizome which is not, strictly speaking, a root, is embedded in the fibrous mat formed by the aerial roots of the tree-fern trunk. The rhizome is not photosynthetic but is covered with minute golden-brown hairs or ‘rhizoids’, and harbours a fungus within its tissues from which it is thought to receive supplementary nutrition. (Fungal hyphae in association with roots are called mycorrhiza.) The rhizome is continuous with the pendant aerial fronds which may reach up to 30 inches but are usually nearer 12 inches. In terrestrial forms the fronds are often semi-erect. The green aerial portion has small scale-like emergences near the base which gradually pass into the larger characteristic leaves. These are peculiar in that they have no distinct stalk but the bases of the ‘leaves’ are applied closely to the stem and spread downwards to give it a ridged appearance. Some writers have likened the leaf-like structures to the flattened lobes of a branching thallus as in the plant-body of a seaweed. The leaf tip ends fairly bluntly but the midrib is often continued forming a little spike or ‘mucron’ on the end of the leaf.
The reproductive organs of Tmesipteris are very interesting. They are two-lobed structures, of spindle-shaped appearance, and are borne on the upper side of a forked appendage. There has been great controversy over the interpretation of these structures. Recent opinion regards them as short fertile lateral branches bearing two leaves and terminating in a ‘synangium’ — a term used for two joined sporangia. The two sporangial chambers are separated by a septum, and the whole structure sheds its numerous spores page 48 by splitting lengthwise. The development of the fruiting body would suggest that there are two sporangia joined, although reduced capsules in which the septum is absent have been found in the frequently occurring ‘abnormalities’. The fertile organs can be borne at any point along the main part of the stem intermixed with the leaves, but there seems to be a tendency for them to be aggregated in fertile zones.
Although not common in New Zealand Tmesipteris can be met with fairly frequently in rain forests throughout the North and South Islands and Stewart Island. It also extends to Australia, the East Indies, north to the Philippine Islands, and to several of the Pacific Islands. Tmesipteris is usually regarded as one species: T. tannensis, as it was first described from material collected at Tanna in the New Hebrides. Tmesipteris means ‘divided fern’, referring to the forked fertile pinnules. The genus has, however, various forms and several species have been suggested. The characters upon which species can be diagnosed are by no means clear; characters vary greatly even on one specimen. Therefore until some satisfactory scheme is worked out, and the cytologists will probably give us this scheme, it would seem better to refer to all the forms under T. tannensis. The varying forms from some of the isolated Islands may later be given specific rank.
Psilotum can be found as an epiphyte but on Rangitoto Island, Auckland, it grows in crevices between rocks. Here the little plants have a ‘twiggy’ appearance like a diminutive flowering broom. The bushes grow up to 18 inches high. The aerial shoots have several equal branchings, the stem is sometimes longitudinally ribbed, and the leaves are reduced to small irregularly placed scales — probably as a response to unfavourable conditions. As in Tmesipteris the reproductive organs are borne in the axils of forked appendages. Here however the sporangial organs are 3-lobed and 3-chambered and are borne on minute bifid leaves. Most of them are carried on the distal portions of the branches.
Psilotum, usually regarded as two distinct species, P. triquetrum and P. flaccidum, is widespread in tropical and sub-tropical countries of both Hemispheres. Only P. triquetrum occurs in New Zealand but its distribution is very restricted. The name, triquetrum, refers to the three-sided stem. It grows on Rangitoto Island, Auckland, and on a few other isolated islands around the coast of the North Island.
Ferns, Lycopods, Horsetails and Psilotales belong to a Division of the Plant Kingdom called Pteridophytes, which have two types of plant in their life cycle. The sporophyte is the obvious plant and culminates in the production of spores in sporangia. The gametophyte arises on germination of the spores and is small and inconspicuous. It produces sexual organs: antheridia giving rise to sperms, and archegonia producing ova. Fertilization of an ovum by a sperm gives a zygote which grows into a sporophyte again.
The Gametophyte Generation
Tmesipteris tannensis growing in humus at base of a tree. Summit, Little Barrier Island.
Miss U. V. Dellow Photo
The gametophytes for both genera are very like the rhizome of the sporophytes; — subterranean brown rhizoid-bearing structures, devoid of chlorophyll, and partially saprophytic because of the mycorrhizal fungus. The antheridia and archegonia are essentially on the fern pattern and are borne intermingled over the surface of the prothallus. The prothalli are cylindrical, occasionally irregularly branched once or twice, and closely resemble a small piece of rhizome. Their measurements range up to 18 mm. in length by 1.25 mm. in diameter. The resemblance to the sporophytic rhizome was shown to be even closer by Holloway's astounding discovery in 1939. He sectioned prothalli from Rangitoto Island and found that some of the larger ones possessed a strand of xylem cells differentiated from the central tissues. To find woody tissue in the gametophyte plant is certainly something quite unique — usually xylem is found only in the sporophyte — and adds another point to the list of puzzling features about these little plants.
Unique Features of the Psilotales
Why is this small order, the Psilotales, so unique among vascular plants? Let us summarise briefly some of the important features:—
(a) The Psilotales are an order showing essentially primitive features. The plants are rootless; the development from the zygote shows that this condition is genuinely primitive and not a result of reduction due to specialization of habit. The plant body is relatively unspecialized anatomically. The sporophyte is in many respects like a primitive simple thallus, the main continuous axis weakly differentiated into underground rhizome and leafy aerial shoot. The gametophyte is likewise a simple branching undifferentiated thallus.
(b) The resemblance of the two generations of the life cycle is more striking in this order than in any other group of Pteridophytes. The vascular gametophyte is unique in the Plant Kingdom.
(c) The Psilotales have no known fossil history in the geological record. Their nearest relatives are a very primitive early Devonian order of fossils, the Psilophytales. It is indeed a puzzle as to why the undoubtedly primitive and ancient Psilotales should have survived without leaving us any faint clue in the fossil record as to ‘what they are or whence they came’.page break
(d) The genera of this order exemplify discontinuous geographical distribution. This occurrence pattern at the ends of the earth probably indicates that the Psilotales are merely relics of a once more continuous and widespread group.
(e) In Pteridophytes the gametophyte generation has the haploid or n. number of chromosomes; the sporophyte has the diploid or 2n. number which is halved by a reduction division or meiosis at spore formation. The cytology of this order is extremely interesting. The chromosome count for the sporophytic cells of the Psilotales is exceedingly high. For Tmesipteris the diploid chromosome number is about 200 (Yeates, 1925), although Manton, 1950, reports a sporophytic number of 400. It seems likely therefore that polyploidy, i.e., multiplication of the diploid number, occurs in this genus. The fact that there is more than one nucleolus in the nuclei of some specimens, established by Yeates, also supports this. Barber of Tasmania has also found a difference in chromosome numbers in material he is now investigating. Okabe 1929, from Japan reported anomalous meiotic nuclear divisions with tripolar instead of dipolar spindles in the related genus Psilotum. It is interesting to note in passing that this type of abnormality occurs in cancer cells and such cytological abnormalities have also been produced when colchicine has been applied to higher plants to induce polyploidy artificially. Barber has correlated meiotic irregularities observed by him in Psilotum with the effect of low temperature. Manton, 1942, observed similar malformation of the meiotic figures in her cytological investigations of Psilotum sporophytes and prothalli which Holloway had sent from Rangitoto Island. She has established that these gametophytes were diploids and the sporophytes tetraploids. The sporophyte number for normal Psilotum is about 100. Cain in his book on Plant Geography makes an interesting observation:— ‘An old or senescent poyploid complex consists only of polyploids. (The diploid members are the first to die out of the complex.) With increasing age the polyploids also begin to die out, so that in the last stages the polyploid complex is simple once more, and is a monotypic or ditypic genus without any close relatives. Psilotum and Tmesipteris are striking examples. Both of these genera are frequently considered to be monotypic and they both have high chromosome numbers. They may represent the remnants of polyploid complexes which flourished hundreds of millions of years ago in the Palaeozoic era. We know from fossil evidence that this group formed a dominant part of the earth's vegetation at that time.’
‘Living Fossils’ indeed! New Zealanders are fortunate in having these interesting little plants in the Flora. We are fortunate too in having them so readily available, for research into the many problems they still present to Botanists. It is to be hoped that New Zealanders will be among those who solve the mysteries of the Psilotales.
Bower, F. O.— Primitive Land Plants. 1935. (For Morphology.)
Bentham, George.— Flora Australiensis, Vol. vii, Page 680. (For discussion of Taxonomy.)
Holloway, Rev. J. E.— The Prothallus and Young Plants of Tmesipteris. Trans. N.Z. Inst., Vol 50, 1918.
Holloway, Rev. J. E.— Further Studies on Tmesipteris. Trans. N.Z. Inst., Vol. 53, 1921.
Lawson, A. A. — The Gametophyte Generation of the Psilotaceae. Trans. Roy. Soc. Edinb. lii, part I. 93-113, 1917.
Manton, Irene — A note on the Cytology of Psilotum with special reference to vascular prothalli from Rangitoto Is. Ann. Bot. London New Series 6. 283-92, 1942.
Manton, Irene — Problems of Cytology and Evolution in the Pteridophyta. Chapter 14 — The Psilotales. C.U.P. 1950.
Okabe, S.— Tetraploide Gartenrasse von Psilotum nudum und die tripolige Kernteilung in ihren Sporermutterzellen. Scientific Report Tohoku Imp. University. Series 4 1929, 373-380.
Sykes, Miss M. G.— The Anatomy and Morphology of Tmesipteris. Annals of Botany, 22, 1908.
Yeates, J. S.— The Nucleolus of Tmesipteris tannensis. Proc. Roy. Soc. London. Ser. B. 98, 1925, 227-224.