44 DIFFERENTIATION OF THE TISSUES 



the mesophyll cells, which correspond to the primary cortex of 

 stems. Near the bases of leaves, below where the vascular 

 bundles have become split up to form the smaller veins, there 

 is often a sheath of cells surrounding the bundles which corre- 

 sponds to the parenchyma cells of the pericycle of stems. 



Where the vascular bundles first enter the leaves they have 

 essentially the same constitution as in stems, with the excep- 

 tion of a functional cambium, but they become smaller as they 

 proceed and ramify throughout the leaf and have correspond- 

 ingly fewer parts. The rule is that the sieve tubes gradually 

 give place to elongated but otherwise undifferentiated paren- 

 chyma cells; and in the ultimate ramifications only spirally 

 and reticularly thickened tracheids are left to represent the 

 bundle, and these are surrounded by a sheath of parenchyma 

 cells belonging to the mesophyll or primary cortex, and known 

 as the parenchyma sheath. It is the function of these meso- 

 phyll cells to collect and carry toward the base of the leaf the 

 foods manufactured by the rest of the mesophyll (see Fig. 90). 



Illustrative Studies 



I. Prepare cross and longitudinal sections of the growing 

 apex of stems of Aristolochia sipho by imbedding the material 

 in parafhn and making permanent stained mounts as described 

 under Cytological Methods in Chapter XIV. Use erythrosin 

 and iodine green for the stains or safranin and haematoxylin. 



Prepare in the same way sections from several successive 

 internodes back from the apex. The object is to follow the 

 progressive development of the tissues from the primordial 

 condition at the apex down to where the primary permanent 

 tissues appear. Find all of the tissues described in the chapter. 



Draw a few cells from each tissue, using the eyepiece scale 

 (page 279) to get all details to scale. Determine the actual 

 sizes of the cells and thicknesses of the walls. 



The erythrosin and iodine green will stain cellulose walls 

 pink, and lignified and cutinized walls green, and in this way 



