Tuatara: Volume 15, Issue 1, May 1967
How to look at Hebe
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.
|K. = Kermadec Is;||Ch. = Chatham Is;|
|St. = Stewart Id;||A. = Auckland Is;|
|C. = Campbell Id.|
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.
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’).page 14
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.