STRUCTURE OF THE ROOT 



79 



divisions of cells iust outside the tip of the cortex. The chief difference is 

 tliat the piliferous layer is distinct in origin from the cortex, while in the 

 Barley the)' were seen to have a common origin. A second t\'pe is illustrated 

 by the Pea and other Leguminosae. {Fig. 6i, B.) Here the stele, cortex, 

 piliferous layer, and root-cap all originate from a common mass of meristem, 

 which occupies the apex, and segregates gradually into the several tissues 

 as the cells mature. Such facts show that no theory of " germinal layers " 

 can have any general application in the development of the plant-body. 



The normal increase in number of roots is by the formation of lateral 

 rootlets, which originate from deeply-seated tissues, and force their way 



Fio. 62. 

 Origin of a lateral root from the pericycle, as seen in longitudinal section of 

 Reseda. In {a) the pericycle has divided by periciinal walls to fomi four layers to 

 which the tissues named' are referable ; the endodermis has yielded. In {b) the 

 formative tissues are clearly recognised. The endodermis {end) has developed as a 

 digesti\'e sac. ( •■: 100.) (.^fter ^'an Tieghem.) 



out of the parent root. Such an origin is described as endogenous, and 

 is in contrast to the exogenous origin of leaves, where the surface-tissue 

 remains continuous over the new growth. The lateral root springs 

 from the cells of the pericycle, usually at a point opposite to one of the 

 protoxylem-groups. If the parent root be cut longitudinally through 

 the point where a lateral rootlet is being formed, the cells of the 

 pericycle opposite the protoxylem will be found in active division (Fig. 

 62, a). Later the tissues thus formed give rise to the central stele [pier), 

 the cortex (cort.), piliferous layer (pHif.), and root-cap [calyp.], of the 



