Leaves and Their Structures 



25 



between the upper and lower epidermis is the mesophyll tissue 

 (Greek: w^^o, middle, and ^/^j/Z, leaf). This tissue is green in 



1/ rX\9 



Figs. 12 and 13. Upper epidermis of "Wandering Jew" (Zebrina) leaf, on the left, and lower 

 epidermis, on the right. St is a stoma, G a guard cell, and Sc a subsidiary cell. The stomata 

 are found only on the lower surface of this leaf. 



color and may also be called chlorench)mia (Greek : chlor, green, 

 and enchyma, tissue). 



The veins consist of three tissues : the water-conducting, food- 

 conducting, and mechanical tissues. The blade, therefore, com- 

 monly contains five tissues : the epidermis, the mesophyll, and 

 three tissues of the veins (Fig. 11). 



How cells are held together. The cells which form the tissues 

 of plants are held together by a layer known as the middle lamella. 

 This layer binds the cells together much as cement binds the 

 bricks in a wall. The middle lamella is usually composed of 

 calcium pectate. If it is dissolved out or changed by chemical 

 action, the cells fall apart, just as bricks fall apart when the 

 mortar between them is dissolved and removed by weathering. 

 In boiling, the pectate between the cells is dissolved, and it is 

 this action that causes fruits and vegetables to break up into a 

 soft mass when cooked. The cells of ripe fruits are also easily 

 pulled apart because of changes in the pectic compounds of the 

 cell wall during the process of ripening. 



