LATERAL ROOTS 71 



serve for the transference of water through the endodermis, 

 which is otherwise impermeable. A thickened endodermis inter- 

 rupted by thin-walled passage-cells is found particularly amongst 

 Monocotyledons (see also Fig. 33, Pa.). 



The mode of origin of the laterals can be readily studied in 

 longitudinal sections through a Bean root. The lateral roots 

 arise by division of pericyclic cells, either opposite the protoxylem 



XJ, 



FIG. 30. The root of a Bean seedling in surface-view and in transverse 

 section showing the origin of the lateral roots (i.r.). C., cortex; 

 ph., phloem ; p.c., pericycle ; r.c., root-cap; xy., xylem. 



strands (Fig. 30, l.r.), or between these and the phloem, so that 

 in Dicotyledons they form vertical series equal to or double the 

 number of xylem groups. In Monocotyledons, owing to the 

 numerous protoxylem-strands, and the early decay of the tap- 

 root, this arrangement is often obscured. In the course of their 

 further development, the side-roots push their way through the 

 cortex, so that, by the time they emerge at the surface, the 

 protective root-cap is fully formed (Fig. 30, r.c.). This so-called 



