172 



GROWTH HABITS OF PLANTS 



INIaximov, a Russian ecologist, believes that solar radiation and an 

 actual shortage of water are the two principal external factors that 

 induce xeromorphic structures. A shortage of water in the em- 

 bryonic tissues prohibits the normal development of cells during the 

 period of enlargement and leads to the production of small cells with 

 thick walls. This decrease in the size of cells leads to the production 

 of small leaves with a close network of veins and closely spaced 



Fig. 77.— Agave scabra, a succulent xerophyto. 



I 



stomata. Intense solar radiation induces the production of palisade 

 tissues. The increase in the amount of palisade tissue increases the 

 capacity for photosynthesis while the closely spaced stomata 

 increases the rate of transpiration per unit area. It has been found 

 by experiment that when a mesophyte and a xerophyte are grown 

 under identical conditions with an abundant water supply and a 



