DIFFERENTIATION OF TISSUES 



293 



154. Roots. True roots appear only in connection with 

 the vascular plants (Pteridophytes and Spermatophytc-) : 



PIG. 274. Section through foot- 

 tip of Pteris: the cell with 

 a nucleus is the single apical 

 cell, which in front has cut 

 off cells which organize the 

 root-cap. CHAMBERLAIN. 



and in all of them the 

 structure is essential- 

 ly the same, and quite 

 different from stem 

 structure. A single 

 apical cell (in most 

 Pteridophytes) (Fig. 

 274) or an apical 

 group (in Spermato- 

 phytes) usually gives 

 rise to the three em- 

 bryonic regions der- 

 matogen, periblem, 

 and plerome (Fig. 

 275). A fourth re- 

 gion, however, pecul- 

 iar to root, is usually added. The apical cell or group cuts 

 off a tissue in front of itself (Fig. 274), known as the calyp- 

 trogen, or " cap producer," for it organizes the root-cap^ 

 which protects the delicate meristem of the growing point. 

 37 



FIG. 275. A longitudinal section through the root- 

 tip of spiderwort, showing the plerome (pi), 

 surrounded by the periblem (p), outside of 

 periblem the epidermis (e) which disappears in 

 the older parts of the root, and the prominent 

 root-cap (c). LAND. 



