DIFFERENTIATION OF TISSUES 



293 



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

 the vascular plants (Pteridophytes and Spermatophytes) ; 

 and in all of them the structure is essentially the same, 

 and quite different from stem structure. A single ap- 

 ical cell (in most Pteridophytes) (Fig. 274) or an apical 

 group (in Spermatophytes) usually gives rise to the three 

 embryonic regions — dermatogen, periblem, and plerome 

 (I^ig-27o). 



A fourth region, how- . ^ P ^ 



ever, peculiar to root, is 

 usually added. The apical 



Fig. 274. Section through root-tip of 

 Tigris: the cell with a nucleus is the 

 single apical cell, which in front has 

 cut ofE cells which organize the root- 

 cap. — Chamberlain. 



Fig. 275. A longitudinal section through 

 the root - tip of shepherd's purse, 

 showing the plerome {.pl\ surround- 

 ed 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). 

 — From "Plant Relations." 



cell or group cuts ofE a tis- 

 sue in front of itself (Fig. 

 274), known as the calyptrogen, or " cap producer," for it 

 organizes the root-cap, which protects the delicate meri- 

 stem of the growing point. 



Another striking feature is that in the stele there is 

 organized a single solid vascular cylinder, forming a tough 

 central axis (Fig-.. 277), from which the usually well-devel- 

 oped cortex can be peeled off as a thick rind. In this vas- 

 ctilar axis, which is called "a bundle " for convenience but 

 does not represent the bundle of Seed-plant stems, the ar- 

 rangement of the xylem and phloem is entirely unlike that 

 37 



