The Growth-Forms of Natal Plants. 
615 
that in moist air, or in feeble light, branches which normally developed as 
spines became leafy shoots. 
Cockayne found the same in the case of Discaria toumatou and supported 
Lothelier's view. 
Zeidler (1911) opposed Lothelier's views. He found that under moist 
conditions and in feeble light thorns were developed in Ulex, and he regarded 
the leafy shoots obtained by Lothelier as being juvenile forms, the thorny 
shoots being the adult. 
In his reply to Zeidler's criticism, Lothelier (1912) defends his former 
position, and states that if the leafy shoots are produced only in moist air 
the latter must be considered as the cause of their production ; and if 
Zeidler's view is correct, the leafy " juvenile " form ought to become spiny 
towards the growing end. This, however, is not so.* 
MacDougall (1912) tested the actual behaviour of the spines of 
Echinocactus with regard to atmospheric moisture. He found that they 
were hygroscopic, and could take up and lose water " as any bit of dry wood 
might do it," but he concluded that the resulting changes in weight of 
Echinocactus, due directly to humidity, do not affect the succulence of the 
living tissues. So that even if we accept Lothelier's views that spines are 
caused by intense illumination and vigorous transpiration, it is not yet clear 
w^hat their exact physiological significance is or how the plant benefits, if it 
benefit at all. 
Another view not necessarily entirely opposed to the above is that thorns 
serve to protect plants against animals. Very little positive evidence can be 
brought forward in support of this view, and the hypothesis is in the main a 
purely teleological one. As Warming points out, "it is evident that spiny 
plants, by reason of their armed nature, may defeat unarmed species, and 
become more widely distributed, but for all this we are not entitled to assume 
that thorns are a direct adaptation to animals, or that they arise in a country 
rich in herbivorous animals." In Arctic countries, however, there are large 
herds of large, herbivorous animals — e.g. reindeer and musk-ox — yet there 
are no thorns, because the conditions of humidity are not suitable for their 
production. 
The only way in which the question of the function of thorns is likely to 
l3e cleared up is by controlled experiments, preferably carried out in the field, 
but certain facts may be noted concerning the thorny species of Natal as 
follows : 
(a) The majority of the thorny species grow in dry situations — e.g. the 
thorn veld, sand-dunes, rocky places. Of these the various species of Acacia 
are typical. 
(b) On the other hand, certain species that grow in the Close Bush under 
* For a review and abstract of Lothelier's 1912 paper, see Journal of Ecology, i, 2, 
p. 128. 
