THE VEGETATIVE STRUCTURES 85 



corky walls are abundant, and they give the leaf a 

 maximum of rigidity combined with elasticity. 



THE ROOT 



No extensive study of the cycad root has been under- 

 taken. There is a main tap root which tapers gradually 

 and extends down to a great depth, so that small amounts 

 of water are brought up even when the surface soil is 

 very dry. Lateral branches are usually small and weak. 

 When plants are growing on exposed rocks where there 

 is httle soil, the root development is remarkable (Fig. 30). 

 In the specimen shown in the figure the upper part of 

 the roots is entirely exposed. Sometimes such roots 

 may extend along the exposed rock for many feet before 

 entering the soil. In an extreme case a root was exposed 

 for forty feet before it finally entered the soil. 



A peculiar feature of the root has attracted much 

 more attention and investigation than the normal 

 structure itself. The small, lateral roots often become 

 infected by " bacterioids " which cause a swelling ac- 

 companied by a profuse dichotomous branching, and 

 besides the roots begin to grow up instead of down, 

 soon emerging above the surface of the soil in dense 

 coralloid masses (Fig. 31). After the infection by 

 bacterioids, a blue-green alga (Anabaena) enters and 

 causes still more distortion. The zone occupied by the 

 bacterioids and alga lies in the cortex between the vas- 

 cular cylinder and the epidermis, where it is easily 

 visible to the naked eye, appearing in a transverse 

 section as a bluish-green ring. The function of these 

 novel roots is problematical. Some have suggested 

 that they may be aerating organs. They are found in 



