192 THE BIOLOGY OF FLOWERING PLANTS 



Epilohium hirsutum ; but not by cabbage, Pelargonium, or 

 sunflower. She ascribes the action of peat in this respect 

 chiefly to lack of aeration. In his memoir on aeration, after 

 a detailed account of the extensive literature on the 

 subject, Clements (1921) concludes that the moor soil is 

 unfavourable because of its extreme poverty in oxygen. 



A different point of view is taken by Warming with 

 regard to the Carices, which he thinks show xerophytic 

 characters inherited from ancestral forms, and which are 

 not always now in accord with their environment. 



There is no doubt that moorland plants are xerophytic, 

 and, in view of the evidence available, it seems highly 

 probable that this habit is a consequence of the action of 

 diverse factors singly or in combination in different cases, 

 though it may turn out that one of these is of more general 

 importance than the others. 



§ 29. Leaf Reduction : Transference of Assimilating 



Function 



Spines.^ — Another form frequently assumed by reduced 

 leaves is that of the spine, an excellent native example being 

 the whin. Here the first two or three seedling leaves are 

 normal and trifoliate — a youth form evidently indicating 

 the ancestral type — but these soon give way to the adult 

 form, a narrow, flat, rather soft, mucronate spine. It is 

 in the axils of these that the angular, branched, and much 

 harder, short-shoot spines arise, on which again leaf spines 

 occur. The well-known needle-like spines of the cactuses 

 may be of leaf nature. Stipular or leaf spines may be 

 formed as well as foliage leaves, as in the false acacia and 

 barberry. In many cases spines are modified shoots, while 

 thorns are epidermal and cortical outgrowths. We may 

 here note that spines and thorns of all kinds reach their 

 most unpleasant degree of development in arid regions of 

 the tropics and sub-tropics, where their development goes 

 along with a greater or less degree of leaf reduction. So 

 notable is this that several communities of both the new 



