DICOTYLEDONOUS STEM 



77 



from that of the root in having strongly thickened outer walls. 

 It forms a continuous layer except where interrupted by the 

 presence of stomata l or lenticels (p. 139) . Its function is to 

 protect the more delicate internal tissues, and in correspondence 

 with this it does not wither at an early stage like the piliferous 

 layer. Occasional cells grow out into hairs (Fig. 34, h.), but 

 these are quite different in character from the root-hairs. 



Immediately within the epidermis are several layers of 

 collenchymatous cells (Fig. 34, s.), thickened mainly on the 



FIG. 34. Photomicrograph of a transverse section of the young stem of 

 the Sunflower (Helianthus). c., cortex ; h., hair ; p., pith ; ph., 

 phloem; r, rays ; s, collenchyma ; xy., xylem. 



tangential walls, whilst between the cells of the inner layers are 

 relatively large intercellular spaces. Such spaces are also abun- 

 dant in the remainder of the ground tissue, and form small tri- 

 angular cavities between the polygonal cells of the pith. 



At the inner limit of the cortex there is a single layer of cells 

 distinguished, by the frequent presence of large starch-grains 

 (Fig. 36, sh.), which can be rendered more prominent by staining 

 with iodine. The layer in question bends outwards around each 



1 For a detailed consideration of epidermis and stomata, see the next 

 chapter. 



