62 



Fruit-growing in Arid Regions 



of leaves. We have seen that plants may be practically 

 stemless, but none of the higher plants can exist without 

 leaves, or organs that take their place. Many of the cacti 

 are leafless, but their stems have been so modified that 

 they perform the work of leaves. 



It is in the leaves and young green parts that plant-food 

 is manufactured; the one element of plant-food that is 



taken into plants in a gaseous 

 state (carbon, in carbon dioxid 

 gas) enters through the leaves 

 and to some extent through other 

 green portions; the excesses of 

 moisture and of gases are given 

 off by the leaves. It will be 

 seen, then, that the leaves are 

 the most important organs in 

 the economy of a plant, and 

 as such are worthy of study. 

 This may be done best by refer- 

 ring to a diagram. 



A cross-section of a leaf is 

 shown in Figure 7, the upper 

 side being at the top of the figure. The first layer of cells, 

 at 1, constitutes the epidermis. The one feature of special 

 importance in the epidermis is the opening or stoma shown 

 at 2. It is through such minute openings that carbon 

 dioxid, with air, enters, and through them the excess of 

 water and of gases is given off. 



Since the water solutions of the soil contain the elements 

 of plant-food in such minute quantities, a much greater 

 amount of water must be taken into the plant than can be 



FIG. 7. Cross-section of a 

 Leaf. (After Stevens.) 



