THE FLORAL ENVIRONMENT 63 



be considered in greater detail in the chapter which 

 considers the relationships between plants and 

 animals in the desert. Here I shall only say that 

 the view that thorns are defensive is difficult to 

 maintain, except in a few special instances. On the 

 contrary, it appears probable that the thorniness of 

 the plants that grow in deserts is due rather to 

 physical than biological influences. It is known 

 that if a plant is grown in a dry atmosphere it 

 produces a relative increase of various hard sub- 

 stances at the expense of its starches and plastic 

 materials ; it is also known that cultivation in a 

 damp atmosphere retards the development of thorns 

 in seedlings. It is therefore possible that the 

 thorniness of desert plants is a purely physical or 

 chemical phenomenon, and that any protection that 

 the plant derives from it is purely incidental. 



