THE GREEN FOLIAGE-LEAF 147 



it is generally differentiated into two distinct parts, namely, 

 (i) the palisade parenchyma which lies beneath the upper 

 epidermis of the leaf, and (ii) the spongy parenchyma which 

 extends between (i) and the lower epidermis. A transverse 

 section across a leaf is given in Fig. 75. The cells forming 

 the palisade tissue are somewhat cylindrical with their long cells 

 at right angles to the surface of the leaf; they have very few 

 intercellular spaces between them. The cells of the spongy 

 parenchyma are very irregular in form and enclose large inter- 

 cellular spaces. 



All the cells of the mesophyll contain numerous chloroplasts 

 but it is in the palisade cells that they are most abundant, a 

 fact which, together with the comparative absence of intercellular 

 spaces, accounts for the upper side of a leaf being usually a 

 deeper green colour than the lower side. 



Ex. 72. — Strip off a piece of the lower epidermis of a bean leaf and mount 

 it in water. Note the irregular outline of the cell-walls and the way in which 

 they fit one with another. Make sketches of these and of the stomata with 

 their guard-cells. Examine, in a similar way, the upper and lower epidermis 

 of the leaves of turnip, plum, apple, onion, grasses and other common plants. 

 Note the form of any hairs which are present. 



Ex. 73. — Cut five or six pieces, each about one-eighth of an inch broad 

 and half an inch long, from the blades of a plum leaf. Place them one on 

 another, hold them in the fingers and cut transverse sections. Mount some 

 of the thinnest sections in water and examine first with a low and then with 

 a high power. 



Sketch the parts seen, namely, — 



(1) The upper and lower epidermis with nuclei, protoplasm, and clear 



cell-sap ; 



(2) The palisade tissue of several layers ; and 



(3) The spongy parenchyma in which are many large intercellular spaces. 

 Possibly the sections of one or more stomata may be seen. 



Ex. 74. — Cut transverse sections through the raid-rib and petiole of several 

 different kinds of leaves. Note and sketch the position and character of the 

 wood and bast of the vascular bundles cut across ; and also the thickness 

 of the walls and nature of the contents of the cells surrounding the bundles. 



Ex. 75. — Prepare some ' Eau de Javelle ' by first dissolving two ounces of 

 carbonate of soda in a pint of water and then adding one ounce of 'bleaching 



