CYCADALES 



"3 



roots. Life found that at the very inception of a branch, bacteroid 

 forms effect an entrance and multiply rapidly in the cells of the apical 



region. The presence of the bacteroids 

 causes a considerable enlargement of 

 the intercellular spaces and a disor- 

 ganization of some of the cells. Algae 

 soon appear in the intercellular spaces 

 and their multiplication still further en- 

 larges them. In its fully developed 

 condition the algal zone, in transverse 



Fig. 94. — Cycas revoluta: transverse 

 section of a "root tubercle," showing 

 the conspicuous algal zone; X20. — • 

 After Life (31). 



section, appears as a conspicuous ring 

 about midway between the stele and 

 the periphery (fig. 94). The alga has 

 been described as an Anabaena, but 

 looks more like a Nostoc, although it is 

 possible that there may be more than 

 one alga. Life' believes that the 

 tubercles serve both in aerating and 

 in assisting in nitrogen assimilation. 

 From a recent study Zach (73) has concluded that a fungus is not 

 the cause of the tubercles, although a mycelium is abundant in them; 



Fig. 93, — Cycas revoluta: neg- 

 atively geotropic roots in coralloid 

 masses, as they appear just above 

 the surface of the soil; about 

 natural size. 



