XKUOI'IIVTE SdCIETlKS 



199 



trate or creeping habit is a luucli less exposed one in such 

 regions tlian the erect haljit. In tlie same manner, the very 

 characteristic rosette liabit, witli its cluster of uverlapping 

 leaves close against tlie ground, tends to diminish loss of 

 "water througli transpiration. 



One of the most common results of xerophytic conditions 

 upon body habit is the development of thorns and spiny 



Fig. 176. Tountr phivit? of E^ipliorh\a splenOtrt^, showing a developmc-Dt uf thorns 

 characteristic of the plants of dry regions. 



processes. As a consequence, the vegetation of dry regions 

 is characteristically spiny. In many cases these spinv ]iro- 

 cesses can be made to develup into ordiiuirv stems or leaves 

 in the presence of more favorable water conditions. It is 

 probable, therefore, that such structures represent reiluc- 

 tions in the growth of certain regions, caused by the unfavor- 

 able conditions. Incidentally these thorns and spiny pro- 

 ces,ses are probably of great service as a protection to plants 



in regions where vegetation is peculiarly exposed to the 

 14 



