114 



MINUTE STRUCTURE OF THE ROOT. 



ductions come, of course, the apparently unbroken rings of liber 

 and the solid masses of wood in old roots. If this development 



of new wood and 

 liber in a perennial 

 dicotyledonous 

 plant proceeds 

 uninterruptedlj-, 

 there will exist at 

 the end of the first 

 year secondary ele- • 

 ments in large 

 amount. After a 

 period of rest, a 

 perennial root re- 

 sumes growth at 

 the points where it 

 was suspended, and 

 the formation of 

 new cork, cortex, 

 liber, and wood goes on as before, until it receives further 

 checks. Owing to conditions to be explained later, the charac- 

 ter of the woody elements is 

 not the same at the begin- 

 ning and end of an active 

 period ; hence there is gen- 

 erally a clearly defined out- 

 line bounding the product of 

 growth of successive years. 



347. More or less of the 

 parenchyma of the original 

 cylinder may remain in the 

 form of radial lines or of 

 bands (medullary rays), 

 some of the same sort of 

 tissue may be subsequently 

 produced from now points of 

 activity, and hence long and short radii will be met with. 



Fig. 94. Section tin-ougli the central cylinder of a binary root of a dicotyledon (Beta 

 vulgaris): e, internal layer of the proper cortex; p, endodermis ; m, peripheral layer of 

 .the cylinder; l^ liber fascicles; v, woody fascicle; c, conjunctive parenchyma (pith and 

 medullary rays.) (Van Tieghcm.) 



Fig. 95. Section Ihrougli the central cylinder of a binary root of a monocotyledon 

 .(Allium Cepa): e, internal layer of the propercortex; jj, endodermis; m, periphei'al layer 

 of the cylinder; 1, liber fascicles; v, woody fascicle; c, conjunctive parenchyma (pith 

 and medullary rays). (Van Tieghem.) 



95 



