8o 



PLAJV7' LIFE. 



beneath the cortex, through which it gradually makes its way 

 by the destruction of the tissues ahead of it, partly through 

 disorganization of the tissues by pressure, and, probably, 

 partly through actual digestion and absorption of the material 

 of these cells. When the rootlet reaches the surface it 

 emerges, therefore, from a distinct rift in the cortex (fig. 94). 



Fig. 93. 



Fig. 94. 



Fig. 93.— Transverse section of a root of a iem(Pteris cretica), passing through the 

 axis of a rootlet which has not yet emerged. Only the stele and three rows of cortex 

 shown, a, apical cell of rootlet, forming anteriorly the root-cap, ep, and posteriorly 

 the body of the root, ec, e, c, pd ; b, binary xylem bundles; /, phloem bundle with its 

 fellow opposite; pe, pericycle; en, endodermis; /, temporary digestive pouch, in 

 course of disorganization and digestion; d, cells of cortex, which will be disorgan- 

 ized as rootlet advances. Highly magnified. — After Van Tieghem. 



Fig. 94. — The same as fig. 93, but older; not quite so much magnified. The rootlet 

 is just emerging from the parent root, pd, c, stele of the rootlet ; ec, its cortex; 

 d, disorganized cells of cortex, ec' , of parent root; b' , secondary xylem; other letters 

 as in fig. 93. — After Van Tieghem. 



96. External conditions. — Branching of the root is often 

 profuse, and is dependent very largely for its character upon 

 the conditions under which it takes place. In those roots 

 which penetrate the soil, it is profoundly modified by the 



