THE STEM AND THE LEAF 



57 



all the year round, and in these photosynthesis is carried on 

 by the green layer of the cortex beneath the epidermis of 

 the stem. 



In many dry 

 countries there 

 are switch plants, 

 which either bear 

 a few little leaves 

 during a small 

 part of the year 

 or are entirely 

 leafless, as is 

 shown in figure 

 37. Like the cacti, 

 such plants, and 

 others with flat- 

 tened stems and 

 little leaf sur- 

 face (fig. 38), do 

 the greater part 

 or all of their 

 photosynthetic 

 food-making in 

 the green cells 

 of the cortex of 

 the stem. 



Many of our 

 common forest 

 trees and shrubs 

 do food-making 

 in the green layer 

 of the bark of 

 the young twigs, 

 until this is shut 

 away from the 

 light by layers /, flowers ; I, leaves ; *, stems. One half natural size 



FIG. 38. Branches of Muehlenbeckia, a plant with 



scanty leaf surface and flattened stems which do 



most of the photosynthetic work of the plant 



