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 di\'ision of pericyclic cells, either opposite the protoxylem 



Fig. 30. — The root of a Bean seedling in surface-%'iew and in transverse 

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

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



strands (Fig. 2'^, I.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 



