DIFFERENTIATION OF TISSUES 



293 



154. Roots.— True roots appear only in connection with 

 the vascLihir phmts (Pteridophytes and Spermatophytes) ; 



P P^ P ? 



Fig. 274. Section through root- 

 tip oiPteris: 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 cahjp- 

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

 which protects the delicate mcristem of the growing point. 

 87 



Fig. 275. A longitudinal section throngh the root- 

 tip of spiderwort, showing the plerome {j)l\ 

 surrounded by the periblem { p), outside of 

 periblem the epidermis (f) which disappears in 

 the older parts of the root, and the prominent 

 root-cap (c).— Land. 



