The Groivth-Forms of Natal Plants. 
617 
is most typical — a short bole and a wide- spreading crown — a growth-form 
adopted in relation to the dry, hot winds, to which the trees are fully exposed. 
This umbrella form is brought about by slow, regular growth, with branching 
at the apex. The lower branches are the oldest, and are not the result of 
vigorous branch development from dormant buds. In such cases those lower 
branches are often destitute of spines and are covered with lichens. This 
umbrella form is typically a more leafy form, and has fewer and smaller 
spines than other forms. Leaf development and spine development, as is 
well known, usually vary inversely. 
In other cases, the species of Acacia show more upright, irregularly 
branched, straggling growth, due largely to more vigorous development. 
Lower branch buds in these forms frequently develop rapidly, forming 
the so-called " gourmand " shoots. On such branches, spines are larger 
and stronger. It is not therefore a case of the lower hranches as such showing a 
greater degree of spinosity, hut hranches that are growing more vigorously. 
This applies also to young plants and coppice shoots and to Aheria caffra, 
when kept as a hedge. 
On the other hand, in such a case as Scolopia zeyheri, the spines (which 
may be absent altogether) are often absent from the crown and present on 
the main trunk 6-8 in. long. Here it is the older part of the tree, which is 
armed. But it is difficult in this case to see what protection this affords to 
the foliage of the tree. 
The evidence, in the main, seems to support Lothelier's view, that spine 
development is the result of dry conditions and intense illumination. The 
thorny species which occur in the Close Bush do not disprove Lothelier's 
view, for, as is pointed out elsewhere (1, 2), the Close Bush of Natal has 
certain xerophytic characters, and is, in fact, intermediate between sclero- 
phyllous woodland and tropical rain forest. The foliage of many of the 
Close Bush trees is of a distinctly xerophytic type, e.g. that of Podocarjms, 
the dominant tree of the Yellow- wood Bush. It is not surprising, there- 
fore, to find some of the species in the Close Bush also possessing thorns. 
"With drier conditions the spinosity undoubtedly increases, and that in a 
most striking manner, until it culminates in the vegetation of the Thorn 
Yeld and dry, rocky hillsides of the Low Yeld region. The production 
of thorns may then be looked upon as one of the visible results of the 
process of lignification, which is characteristic of xerophytes, the physio- 
logical significance of which is still somewhat obscure, but which the 
researches of Lothelier, as well as the evidence presented by the growth- 
forms and distribution of the species of plants concerned, have shown to be 
favoured by dryness of the atmosphere and intense illumination. 
Accepting this, we may still ask whether, being produced in this way, 
they may not then serve as a protection against animals. This is a very 
different position from supposing that thorns have ]ieen evolved as a protec- 
