738 



ECOLOGY 



tallest individual forms, Sequoia and Eucalyptus, occur in much drier 

 regions. Maximum diametral increase occurs, on the whole, in rela- 

 tively arid climates. It is sometimes stated that the baobab, a xero- 

 phytic African tree, has the greatest diametral growth of all known 

 trees; other xerophytic trees with great diametral enlargement are the 

 dragon tree (Dracaena Draco) and Cavanillesia. However, some 

 trees of great diameter (as the redwood and the hemlock) are meso- 

 phytic. 



Probably trees are a relatively recent product of evolution, and 

 doubtless the tree habit has originated many times and in many habi- 



FiG. 1060. The mountain sheep (Raoulia eximia), a remarkable alpine composite, 

 illustrating the culmination of the cushion habit among seed plants; each coral-like colony 

 is composed of thousands of separate but closely compacted shoots, the whole mass being 

 attached to solid rock by a single root system; New Zealand. From COCKAYNE. 



tats. Sometimes it has been assumed that the tree habit is a result 

 of the " struggle for existence " in mesophytic climates; this, per- 

 haps, is a tenable view, since growth in dense cultures increases stem 

 elongation. However, there is equal reason for believing that trees 

 have arisen also in dry regions, since lignification and diametral en- 

 largement are best developed there. Perhaps the most essential step 

 in the evolution of the tree habit is in the passage from an herb to a 

 shrub; if so, experiments on such plants as Spiraea salicifolia and 



