72 NEW ZEALAND PLANTS. 
where the plant has reverted to the juvenile stage. The whipcord 
koromikos exhibit the phenomenon under discussion to an extreme 
degree. The juvenile has extremely thin leaves, which could not 
tolerate desiccation; whereas in the adult the leaves are reduced 
to thick scales, which are pressed close to the stem, and thus 
adapted to resist drought. 
Experiment shows that in some cases the juvenile and adult 
stages are dependent upon the environment, and can be produced 
or suppressed. Cultivation in moist or in dry air is one agent 
which brings about the change. The juvenile spineless wild-irishman 
(Discaria toumatou) can be kept for many years in that condition by 
moist-air culture; but a week or two in dry air and the spines 
appear. In a similar manner, leaves can be called forth on the 
leafless common New Zealand broom (Carmichaelia subulata); the 
whipcord veronicas can be made to put forth true, thin leaves 
(fig. 39); and the strongly concavo-convex leaf of the boat-leaved 
tree-daisy (Olearia cymbifolia) can be flattened out. 
The extreme plasticity of the manuka (Leptospermum scoparium), 
referred to in Chapter II, must suggest to any one that this species 
can get on very well in the world. This is true enough, for the 
plant in question can live and enjoy the best of health in forest, in 
swamps, on dry rocks, on river-bed, on cold subalpine moorlands, 
on sandhills, near the seashore, and exposed to sulphur-fumes near 
hot springs. 
The manuka produces yearly its wealth of snowy flowers in the 
greatest abundance. Their white colour renders them exceedingly 
conspicuous. They are more or less bisexual, even on the same 
branch, and so depend to no small extent for pollination upon insects 
in search of their honey and pollen. The seeds are minute, pro- 
duced in great profusion, readily germinate, and so light that they 
are easily blown up from the ground along with the dust. Fire 
does not destroy the seeds to any extent, enclosed, as they are, in 
their stout woody capsules. If the above facts are borne in mind, it 
would seem remarkable indeed if the manuka did not form a most 
widespread plant-association of its own, which was not only capable 
of holding its place against the attacks of man, his grazing-animals, 
and fires, but could increase and become extremely hard to get rid 
of. It may be pointed out that Leptospermum scoparium itself is 
not composed of a definite set of individuals forming one biological 
