﻿EXISTING AND FOSSIL CYCADS COMPARED. 



207 



THE ROOT. (Fig. 119.) 



The cycads, unlike all vascular cryptogams, send down a primary root which 

 continues as a tap-root and may be large and prominent, approaching the diameter 

 of the trunk itself, as shown in the accompanying figure 119 of Dion edule. In 

 subterranean trunks such as those of certain species of Zamia, the tap-root remains 

 distinct and its lateral branches are relatively small, the trunk even assuming, 

 as already noted in the description of Zamia floridana, a carrot-like form. But in 

 most genera the root system comes to be quite filamentous, being largely made up 

 of freely branching secondary or adventitious roots extending in every direction 

 and often reaching the surface of the ground and spreading about in felt-like masses. 

 The manner of branching in these secondary roots, we are informed, is only appar- 



Fig. I 18. — Encephalartos Ghellinckii. > ,',,. A South African cycad with moder- 

 ately persistent armor. The exceptionally long and slender pinnules give the (ronds 

 a distinctly feathery appearance. 



eutly dichotomous as the result of the checking of the activity of the true apex. 

 But it is also possible that a copious monopodial branching like that of the Mar- 

 attiacete may in lesser degree be present in some of the cycads. According to 

 De Bary the cycad root cap, as in other gymnosperms, arises from the splitting off 

 of the outer layers of the periblem covering of the meristematic region, there being 

 no true calyptrogen or dermatogen, as in the angiosperms. The "coral-like" or 

 bushy dichotomous rootlets seen about the bases of most Cycas stems are an 

 excrescent growth and may incidentally contain Nostoc forms in the cortical zone. 

 The first cause of growth disturbance and excrescence is doubtless explained by the 

 presence of bacterioid forms, which invade even the initial apical cells of the 

 rootlets (78), although no microscopic structure has been observed directly com- 

 parable to that of leguminous tubercles. 



