THE VEGETATIVE STRUCTURES 69 



ground, but in others even the bud is below the surface. 

 The armor of leaf bases is likely to be poorly developed 

 or entirely lacking in these underground stems. 



For various reasons the columnar type of stem has 

 received more attention. It is more conspicuous, can 

 be seen without digging, its armor of leaf bases is an 

 interesting feature, and it is the type which has been 

 retained from its fossil ancestry. 



The tallest of all cycads is Macrozamia Hopei, of 

 northern Queensland, which occasionally reaches a 

 height of sixty feet; Dioon spinulosum, with an occa- 

 sional specimen fifty feet in height, comes next; and the 

 Cuban Microcycas, with here and there a specimen over 

 thirty feet in height, is third. But these are all excep- 

 tional figures, even for these species; very few of them 

 get beyond twenty feet. Among the remaining columnar 

 forms any plant reaching a height of ten feet must be 

 regarded as very tall. 



The leaning habit, shown in Fig. 18, is common 

 among the taller specimens, outside of the three very 

 tall species just mentioned. The explanation is not 

 hard to find. The root system is not very extensive, 

 and the plants are very heavy; consequently, as soon as 

 they become tall enough to be much affected by the wind 

 they begin to lean and finally become prostrate. The 

 main root seldom breaks, but the tissues at the base of 

 the trunk become ruptured, and from these wounded 

 portions buds arise and develop into new plants. Gar- 

 deners propagate some species of cycads by wounding 

 the stem and thus causing the development of buds 

 which can be potted. The bud at the apex of the pros- 

 trate trunk may become erect and produce crown after 



