THE LIVING PLANT 



31 



FIG. 18. Portion of a cross-section of a leaf showing 

 different cells. 



by a single layer of cells known as the epidermis (Fig. 18). 

 In the epidermis are small openings called the stomata. 1 

 Each stoma lies between two modified cells, called guard-cells. 

 These open and 

 close, thus regu- 

 lating the size of 

 the stoma. Un- 

 derneath the 

 upper epidermis 

 are one or more 

 layers of cells, 

 which, because 

 of their long 

 shape , are 

 termed palisade- 

 cells. These con- *$3fcww 

 tain chloroplasts 

 and chlorophyll, 

 a green colored substance. Beneath the palisade-cells are 

 irregular-shaped thin-walled cells (parenchyma cells) and 

 between them are air spaces (intercellular spaces). 



47. The functions of leaves (Fig. 19). Leaves are very 

 important plant organs. One function is that of transpira- 

 tion, or the " giving off " of water in the form of vapor. Leaves 

 give off moisture for two reasons. In the first place, transpi- 

 ration regulates the temperature of the plant. Secondly, the 

 plant takes up more water than it can use in order to obtain 

 the necessary amount of soluble food materials. It must get 

 rid of the surplus water, which it does through the little open- 

 ings (stomata) in the epidermis of the leaves. The guard- 

 cells resemble in shape the halves of a doughnut. When 

 there is plenty of moisture, the guard-cells are full of water 

 and are turgid so that between them there is a wide opening. 

 If for any reason moisture is lacking in the plant, transpiration 



1 Singular stoma. 



