THE FUNDAMENTAL TISSUES 



133 



FIG. 96. Rootstock of the bracken fern 



distinct, more or less continuous, zones: an outer one lying imme- 

 diately under the epidermis and somewhat interrupted opposite 

 the angular projections on the sides of the rhizome, and an inner 

 one coming between 

 the large medullary 

 and the smaller ex- 

 ternal series of 

 bundles. The stor- 

 age cells, which are 

 characterized in the 

 figure by their walls 

 and in nature by 

 the large amount of 

 starch present in 

 their cavities, oc- 

 cupy all the trans- 

 verse section not 

 taken up by the epi- 

 dermis, the fibro- 



vascular system, and 

 the sclerotic bands 

 just described. The 

 mechanical require- 

 ments of the stems 

 of many lower vas- 

 cular plants are pro- 

 vided for by skeletal 

 structures present in 

 the fundamental 

 system. This situa- 

 tion is common to 

 both the Pteropsida 

 and the Lycopsida. 



. 97. btem of Lepidodendron Spencen , , , .. 



The truth of this 

 statement is revealed by the figure presented of the stem of a 

 lepidodendrid, a very ancient representative of the group of club 

 mosses or Lycopsida. Here zones of thick-walled skeletal tissues 



