616 Transactions of the Uoyal Society of South Africa. 
moist conditions are thorny also — e.g. Xanthoxylo^i capense, Scolopia zeyheri, 
and various lianes. In the case of the latter the thorns may be considered 
as organs modified to assist in climbing, though it is doubtful whether this 
explanation applies in all cases. 
(c) Certain of the more accommodating species occur both in the Close 
Bush and also in the dry Thorn Yeld. In such cases there is usually a much 
greater development of thorns in the latter. Sometimes a species may be 
thornless in the Close Bush and possess long spines in the Thorn Yeld — e.g. 
Celastrus huxifolius. 
(d) In the same way species are sometimes more thorny on the sand- 
dunes than elsewhere. In other cases, however, the foliage is more succulent 
in this situation and less prickly — e.g. Xantlioxylon capense. It is worthy of 
note that Scolopia zeyheri, according to Sim (21), is usually thornless on the 
sand-dunes. 
(e) Though in the individual species increased thorn development with 
increased aridity is the rule, there is often a certain variability in thorn 
development which does not seem to depend on that factor or on more 
intense illumination. Young plants of Acacia are sometimes — in fact, one 
might say as a rule — very thorny, and become less so as they grow. A 
passing reference may be made here again to Zeidler's theory. He supposed 
that leafy shoots of Ulex are " juvenile " ; thorny shoots " adult." The 
Natal Acacias show exactly the opposite phenomenon. Juvenile plants are 
very thorny indeed ; adult ones less so. Frequently there is a large thorn 
development on coppice shoots. These alone may be thorny and the rest of 
the tree unarmed, as e.g. in Piectronia mundii, P. pauciflora, Aheria tristis, 
etc. ; or the spines in the coppice shoots may be more abundant and much 
larger than those on other parts of the tree. 
Aheria caffra is often thornless in the tree form, but very thorny if kept 
cut as a hedge. Marloth is quoted by Warming (23) as having called 
attention " to specialised adaptation exhibited by certain species, in that the 
longest and strongest spines occur on young individuals or on root shoots 
which are most accessible to animals, while the branches subsequently pro- 
duced on tall trees are quite devoid of spines." Warming further adduces 
the case of Ilex aguifolium, the upper leaves of which are usually not prickly 
when once the plant has grown into a tree. 
Here, then, we seem to have certain facts which support the " protection 
against animals " theory. But like many other cases, where a theory has to 
be supported simply by observed facts in Nature and not by controlled 
experiments, the amount of support given to the theory depends largely on 
how the facts are presented. In the case under discussion, certain other 
facts must be considered, to reach a fair conclusion. In the various Acacias, 
the dominant trees of the Thorn Yeld, there is a considerable variation in 
growth-form, as well as in degree of spinosity. The " umbrella " form 
