New Zealand Plants and their Story
Prolonged Juvenile Forms of New Zealand Plants
Prolonged Juvenile Forms of New Zealand Plants.
If the seed of Veronica cupressoides, or of any one of the "whipcord veronicas," as they are aptly designated, be sown, it will quickly germinate and produce a young plant, altogether distinct from its parent. In the old plant the leaves are represented by green scales pressed closely against the stem; they are also thick, and have a peculiar anatomical structure. In the seedling, on the contrary, there are true leaves with, a stalk and blade, which are quite thin and of an anatomical structure absolutely different from that of the adult. In other words, the juvenile and the adult plants might be two different species, each adapted for a quite different mode of life, the adult for an arid climate and the juvenile for a moist forest region.
If we are in a position to carry on our investigations a little further, and to grow some of the seedlings in the open air and others in a glass case so constructed as to always contain air saturated with moisture, the plant in the open will by degrees assume the adult and dry-climate habit, while the other will remain in the juvenile and wet-climate form, not for a week or two merely, but for years; indeed, so long as it is kept in a "moist chamber" it will remain a juvenile plant.
And now for a third experiment. Take a rooted cutting of an adult piece of the veronica, and place it in the moist chamber. After a few weeks its new growth will be of the juvenile form, and juvenile and adult leaves will be on the plant at the same time (fig. 24). Similar experiments with certain of the New Zealand brooms (Carmichaelia) and with the wild-irishman (Discaria toumatou) will lead to a similar result. To inquire into this matter at length would be out of place here, but any general account of New Zealand plant-life would be most incomplete without some reference to this extraordinary phenomenon. A few analogous examples may throw a little light on the subject. Many coprosmas and other shrubs belonging to diverse families* have a curious habit of growth, which makes them outwardly so similar that they are not easy to be distinguished when page 61not in flower or fruit. Their branches are stiff and wiry, and interlace one with the other into a dense mass. A similar form is assumed by trees and shrubs subject to constant wind, especially if combined with a dry station.
Fig. 24.—Young Plant of Veronica lycopodioides, which had in part assumed the adult form with scale-leaves, putting forth true leaves in consequence of cultivation in moist air and rather feeble light.
[Photo. L. Cockayne.
Sow the seed of Sophora grandiflora. Again comes the small, erect plant; but this is succeeded by no shrubby, dense form: the young plant continues its development without noticeable change until it is fully grown into a tree. Finally, sow seed of Sophora prostrata. Again the upright early seedling appears, then the juvenile shrubby stage, as in S. microphylla; but this time it never develops into a tree, but has this shrub stage as its adult form. Finally, without going into further details, there are about two hundred New Zealand plants which have adult forms differing considerably from the juvenile, many of which can, when fully grown, revert to the early form.
Now, it is held by certain biologists than an organism in its individual development passes, more or less completely, through the stages assumed by the species at various times during its past evolutionary history. If this is so, then juvenile stages represent ancestral stages, and such a plant as the seedling whipcord veronica, with leaves of the above-mentioned experiment, is the ancestor of the present river-terrace V. cupressoides, artificially brought back into the world, and kept so long as the moist-air treatment continues. This treatment, too, shows that the ancestral plant lived in a much moister climate than is New Zealand at the present time. The behaviour of Sophora, too, throws some light on the matter. The very earliest stage would be the ancestral form; but as this is the same in both S. grandiflora and S. microphylla, the former is ancestral from beginning to end of its development, whereas the latter, in its middle stage, exactly resembles S. prostrata.
All this points out that there came a period of drought in New Zealand, or of a climate requiring drought-resisting adaptations. Then certain adaptations against excessive dryness came into being, as in the cases of the shrubby form of Sophora and the adult of Veronica cupressoides. On a more recent change to a wetter climate some individuals of the former genus would grow out of this drought-resisting form, others would remain unchanged; but in the case of the veronica no reversion has taken place, but the "ancestral form" still remains latent.