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Tuatara: Volume 15, Issue 1, May 1967

How to look at Hebe

page 10

How to look at Hebe

Hebe is credited with more species than any other genus of New Zealand plants, and is reputed to be almost the most difficult. The simple, mostly entire leaves offer few obvious characters to distinguish one species from another, and the small flowers are a good deal alike and are nearly all white. Yet the species differ far more than do sheep on the hill which the farmer recognises individually! It is a matter of training one's eye by looking, consciously or unconsciously, at those features that offer the strongest and most reliable contrasts.

The present notes add little to the scheme already available (Moore in Allan Fl. N.Z. 1961, pp. 885-887) but some learners may find pictorial representation helpful. Diagrams give a chance also to suggest possible relationships. The comparisons are extended beyond Hebe by including parallel information about Pygmea and Parahebe, two genera which, like Hebe, have been split off from the older genus, Veronica.

The seventy-nine New Zealand species of Hebe are arranged in ten groups, the largest group including 21 species, the smallest composed of a single one. Members of each group share certain common features, or vary within a limited range, and in assigning a specimen to its proper group one learns to recognise contrasts within the genus. A summary is presented here in the form of four sets of diagrams, each set with twelve insets, ten of them representing the groups of Hebe, the other two showing Pygmea and Parahebe.

The first set of diagrams (Fig. 1) gives the names of the groups and the number of species in each, and indicates the broad distributional pattern. Approximate altitudinal range is suggested by the terms low, mid, and high. A square inset shows that the group lacks North Island representatives, while an eight-sided one shows that at least some of the included species are found in the North Island. No group is entirely absent from the South Island. Occurrences outside the two main islands but still within the New Zealand Botanical Region are indicated as follows.
K. = Kermadec Is;Ch. = Chatham Is;
St. = Stewart Id;A. = Auckland Is;
C. = Campbell Id.
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Fig. 1: Hebe groups, Pygmea and Parahebe: Names, distributions, numbers of species.

Fig. 1: Hebe groups, Pygmea and Parahebe: Names, distributions, numbers of species.

Distribution beyond the New Zealand region, not shown in the diagrams, involves only two of these groups of Hebe. In South America there are two species of ‘Apertae’ (indistinguishable from South Island plants of H. salicifolia and H. elliptica) and one of these (H. elliptica) extends to the Falkland Islands. On Rapa Island, west of Pitcairn, is found an endemic species belonging to ‘Occlusae’; it resembles Chatham Island species closely, but shows less affinity with any species of New Zealand proper.

The second set of diagrams (Fig.2) depicts leaf bases. In Hebe leaves are always in opposite pairs, and it is characteristic of the genus that the leaf bud at the tip of the shoot is large; the two outermost leaves of the bud are almost full grown before they break apart (well illustrated by photographs in Wardle, N.Z. Jl Bot. 1, 1963: 34). The closed leaf bud gives a good clue to the group to which a plant belongs. First identify the midribs and the margins page 12
Fig. 2: Hebe groups, Pygmea and Parahebe: Bases of leaf buds.

Fig. 2: Hebe groups, Pygmea and Parahebe: Bases of leaf buds.

of the two outermost leaves of a bud and then, using a lens if necessary, determine whether the two margins are in contact throughout, or whether there is a gap (or sinus) between them at the base. ‘Occlusae’ and ‘Subcarnosae’ (top right) show closed buds, ‘Apertae’ and ‘Subdistichae’ (top row, middle and mid-left) show sinuses of ‘gothic’ outline, ‘Buxifoliatae’ (middle below) has a more shield-shaped sinus. ‘Paniculatae’ and ‘Grandiflorae’ illustrate leaf pairs that diverge early instead of remaining together in a tight bud, and in ‘Flagriformes’ and ‘Connatae’ the leaves, which are mostly small, remain connate at their bases throughout life.
Figure 3 shows diagrammatically some features of capsules of Hebe, Pygmea and Parahebe. The capsule valves are shown in outline transverse section; the septum between the two loculi is placed vertically and the floral bract and axis are shown in the conventional way by a triangle at the left and a dot at the right page 13
Fig. 3. Hebe groups, Pygmea and Parahebe: Capsules in transverse section.

Fig. 3. Hebe groups, Pygmea and Parahebe: Capsules in transverse section.

of each capsule. At dehiscence there is always some divergence of carpels along the septum as well as some splitting of the locule wall. A point to notice is the relative length of the septum. In most groups it is long and the capsule is flattened and narrow between axis and bract, but in ‘Grandiflorae’ and ‘Semiflagriformes’ the septum is short and lies at right angles to the plane of compression. In several groups there are species in which the capsule is plump and little flattened in either direction. This diversity discounts the oft-repeated suggestion that capsule form always provides a sure criterion to separate Hebe from Veronica, a short septum being characteristic of the latter genus.

Figure 4 illustrates inflorescence types. The diagrams also show the three leaf forms that result from the types of buds shown in figure 2, i.e. petiolate (as in ‘Buxifoliatae’), sessile (as in ‘Occlusae’) and connate (as in ‘Flagriformes’ and ‘Connatae’).

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Fig. 4: Hebe groups, Pygmea and Parahebe: Inflorescences — positions and types.

Fig. 4: Hebe groups, Pygmea and Parahebe: Inflorescences — positions and types.

A first point to notice about an inflorescence is whether it is strictly terminal or wholly lateral. In the latter case the apical leafy shoot (shown by an arrowhead) is available to carry on vegetative growth above the inflorescence. If the inflorescence is terminal, new leafy shoots can come only from axillary vegetative buds below the inflorescence. When flowers are clustered near the end of a twig, some care is needed to be sure whether or not there is a leafy tip hidden between them.

The inflorescence is racemose though occasionally the pedicels are so short that it appears to be a spike. The least elaborate is that seen in ‘Buxifoliatae’ and ‘Flagriformes’ where the bracts are opposite and almost as large as the leaves, so that the flowers can almost be regarded as solitary and axillary. In species or whole groups where the bracts are smaller and less leaf-like they show an increasing tendency to lose their opposite arrangement — a tendency page 15 culminating in ‘Apertae’ and ‘Occlusae’ where the long many flowered lateral raceme is strongly differentiated from the leafy twig. Branching of the inflorescence gives rise to panicles, mostly lateral as in ‘Subdistichae’, but strictly terminal in ‘Paniculatae’; each of these groups includes stages from simple racemes to panicles. The solitary, apparently axillary flowers seen in Pygmea and in some species of Parahebe are each associated with two, usually opposite, bracts and can probably be regarded as representing much reduced racemes. Such a very simplified inflorescence has not been seen in Hebe.

On the basis of these and other similarities and contrasts, possible relationships between groups are suggested by connecting lines in the diagrams, and in part also by the relative positions of the insets. If the original ‘protohebe’ is imagined as something not unlike Hebe odora in ‘Buxifoliatae’, the present representatives could be derived by gradual specialisation of the reproductive shoots. As bracts are more and more different from leaves in size, texture, and arrangement, the inflorescence is correspondingly more isolated into lateral racemes or terminal panicles. Minor changes can be induced experimentally. It has been noted that in rather pampered garden plants of H. colensoi and H. vernicosa (group ‘Subdistichae’) the usually vegetative tip tends to become reproductive and a number of branches in the bush then end in panicles. In other species in the same group, garden conditions seem to induce increased branching in lateral inflorescences.

For more than a hundred years artificial hybrids have been made between species belonging to widely different groups of Hebe but results have rarely been precisely recorded. A systematic programme of crossing might rather quickly indicate whether appropriate weight has been given to the characters here used to group species together and to differentiate between the groups. Obviously there is still much to be learned from looking hard at Hebe.

* Based on a paper read at the Ninth N.Z. Science Congress, Wellington, 1960. Diagrams prepared at that time by K. R. West from sketches by L. B. Moore.