24 ELEMENTS OF BOTANY. 



We have already seen that leaves are composed of cellular and woody 

 tissue, and have considered the latter in its ramifications which make up 

 the leaf-skeleton. We will now briefly examine the cellular tissue. 



Unlike the cellular tissue of the stem, 

 this is a green pulp closely resembling the 

 green layer of the bark. It is made up of 

 cells somewhat loosely arranged, with open 

 spaces or air-passages between them (Fig. 

 35). These cells owe their green color to 

 minute grains of a peculiar green coloring 

 matter, termed chlorophyll, which they 

 contain. Externally the entire leaf is covered 

 with a thin, transparent membrane, termed 

 epidermis (Fig. 36); this is perforated 



FIG. 37. surface of a leaf, showing sto- w ^ numerous openings, termed stomata 



mate (breathing-pores). Magnified. . . 



(Fig 37), which permit the external air to 



have free access to the intercellular air-passages. The stomata are much 

 more numerous on the under than the upper side of the leaf, and here 

 also the air-passages are most abundant. 



FUNCTIONS OF THE LEAVES. 



Leaves have often been compared with the lungs of animals, since it is 

 their office to aerate the vital fluids of plants. The nourishment collected 

 by the roots is transmitted through the stem to the leaves, and here, ex- 

 posed to contact with the air, it becomes elaborated and fitted for the 

 plant's further use. Through the multitude of stomata, or breathing-pores, 

 the air has free access to the interior of the leaf, where the cells take from 

 it carbonic acid and yield up their superfluous moisture, or absorb oxygen 

 and water as may be required. In sunlight leaves absorb carbonic acid 

 and give out oxygen ; in darkness the process is reversed and carbonic 

 acid is exhaled. But as plants are much more active in daylight than in 

 darkness, the amount of carbonic acid taken from the atmosphere is many 

 times greater than that which is exhaled ; and as nearly all the carbonic 

 acid absorbed is decomposed, the carbon alone being retained while the 

 oxygen is returned to the air, it at once becomes evident that plants are 

 continually purifying the air which animals breathe. Animals, on the other 

 hand, are as constantly renewing the supply of carbonic acid in the air, 

 and thus better fitting it for the sustenance of plants, so that there is an 

 intimate interdependence of vegetable and animal life. Both probably 

 had their advent upon earth at the same time, and progressed upward 

 from the lowest to the highest forms, side by side, with equal steps. 



So far the leaves are analogous to the lungs of animals, but their func- 

 tions do not cease with the mere absorption of carbonic acid and the ex- 



