304 PEACTICAL BOTANY 



are thin- walled ; but, passing inwards, there is a sudden 

 increase in thickness of the wall, so as to form a dense 

 sclerenchymatous ring : this surrounds 



3. The endodermis, which consists of a single 

 layer of cells flattened tangentially, and having the 

 usual dotted marking of the radial walls : this may be 

 difficult to observe, as the radial walls are often pressed 

 out of shape. Within this layer lies 



4. The pericycle, which usually consists of 

 two layers of cells, with thin walls, and obvious proto- 

 plasmic contents. The vascular tissues inclosed within 

 these layers are arranged according to the ordinary 

 radial type ; thus there will be seen 



5. Two groups of xylem abutting on the pericycle 

 and composed of tracheides of various size, the 

 largest being near the centre of the root : as the root 

 develops, the two originally separate groups of xylem 

 unite at the centre by formation of fresh tracheides, 

 and together form a flat band which traverses the 

 root longitudinally. Alternating with the groups of 

 primary xylem at the periphery of the vascular 

 cylinder are 



6. Two groups of phloem, consisting mainly of 

 sieve-tubes having the same characters as those of 

 the stem. Scattered among the vascular elements are 

 cells of conjunctive parenchyma. 



Note that one or two cells of the endodermis opposite the 

 groups of xylem are larger than the rest : these are the rhizo- 

 genic cells, which might have been the starting-points of lateral 

 roots : the latter are formed at an early stage of development of 

 the tissues of the root, i.e. near to the apex ; if transverse sections 

 be made through the young part of a root, lateral roots may be 



