Metaphyta L ilium. 



177 



FIG. 93. ORIGIN OF SECONDARY 

 ROOTS. (Prantl.) 



and moisture. The root is the first part of the embryo to 

 make its way through the micropyle and elongate. The 

 plumule or young stem remains hidden for some time in the 

 seed. When the primary root has taken fast hold of the 

 soil the secondary roots begin to make their appearance to 

 take its place functionally, and the primary root soon ceases 

 to elongate. The plumule next emerges and increases in 

 size. The stems of plants of the lily type increase in thick- 

 ness very rapidly, but do not add secondary wood in future 

 years if perennial. There are 

 a number of arborescent lilies 

 such as the Yucca, Dracaena, 

 and such like which, how- 

 ever, form interesting tran- 

 sition links to the dicotyle- 

 donous type which we shall 

 discuss in the following sec- 

 tion. One point of some im- 

 portance remains to be noticed 

 with regard to the mode of 

 origin of lateral branches of 

 the stem and of the root axis. 

 Lateral roots always originate 

 from what was described at p. 

 150 as the pericambium layer, 



, , secondary roots in various stages 

 of growth ; k, root-cap ; r, cortex ; 

 f, fibro-vascular core. 



especially that portion of it which lies opposite to the xylem 

 of the fibro-vascular strands, whilst branches of the stem 

 axis originate from superficial layers of cells of the cortex 

 into which the fibro-vascular strands afterwards become 

 developed. The root method of development is termed 

 endogenous, whilst the shoot method is termed exogenous. 

 One result of the endogenous mode of origin is that the 

 roots have to push their way through the cortex of the root ; 

 hence, where each root appears on the surface, it is sur- 

 rounded by a collar of tUsue which it has pushed aside, the 

 coleorhiza. /^ 



N 



