RELATION OF LEAF TO PHOTOSYNTHESIS 145 



has a waterproofed outer wall after the manner of a typical 

 epidermis (see page 28), and it is perforated on one or both 

 surfaces of the leaf with stomata to allow the entrance of carbon 

 dioxide as described in Chapter VIII. The ground parenchyma, 

 also called the mesophyll, is differentiated into three kinds of 

 tissues: the palisade parenchyma, occupying the upper half; 

 the spongy parenchyma, of the lower half; and the border paren- 



FiG. 77. — Showing intercellular spaces: /, between the palisade cells; e, in a leaf; g, 

 border parenchyma; h, tracheal elements of the vein. Camera-lucida drawing of a tangen- 

 tial section of a leaf. 



chyma, forming a sheath around the vascular bundles in the 

 veins (Fig. 78). The border parenchyma cells deliver water 

 from the veins to the palisade and spongy parenchyma, and they 

 gather food from these and conduct it to the sieve tubes in the 

 larger veins, as described in the next chapter. 



The intercellular spaces in the spongy parenchyma are much 

 larger than those in the palisade parenchyma, and they freely 

 communicate with one another and with the intercellular spaces 

 in the palisade parenchyma, and so are well suited to receive 

 and distribute the carbon dioxide which enters mostly through 

 the under surface because most of the stomata are there as a 

 rule. The intercellular spaces in the palisade parenchyma 



