THE LEAVES. 135 



and are sensitive to various external conditions, especially 

 light, so as to control the size of the slit-like space between 

 them by changes in their curvature (fig. 162). This slit is 

 formed, like most intercellular spaces, by the partial splitting 

 apart of the cells. It communicates with extensive intercel- 

 lular spaces in the interior. 



The stomata are very numerous. In different plants, in 



the space here enclosed 



sq. cm. 



, the numbers usually vary 



from 4000 to 30,000, sometimes, however, reaching as much 

 as 60,000 to 70,000 in the olive and rape. They are not 

 equally distributed on the two sides of the leaf, being usually 

 more numerous on the under side, where there are more in- 

 ternal intercellular spaces. They may be wanting on the 

 upper side, as in lilac, begonias, and oleander. There are 

 no stomata on submerged leaves nor on the under sides of 

 floating leaves. In some plants they are found in clusters, 

 in others uniformly distributed. 



167. Cortex.- -The cortex of leaves is called the meso- 

 phyll. It consists of thin-walled, active cells, for the most 

 part richly supplied with chloroplasts. In thick leaves the 

 internal cells are without them. In some leaves the cells of 

 the mesophyll are nearly uniform, but in most those near the 

 upper surface are more elongated and close set, forming one 

 or two rows, with their ends outward, while cells near the 

 lower surface are irregular in form, with large intercellular 

 spaces. These tissues are known as the palisade and spongy 

 parenchyma (fig. 163). 



About the steles, the cortex forms the usual endodermis 

 (gs, fig. 163), and often develops along the larger into one 

 or two strands or a sheath of mechanical tissues. These 

 tissues, together with a stele, constitute the rib or vein, often 

 so massive ns to project beyond the other parts in thin leaves. 



