THE NUTRITIVE ORGANS OF PLANTS 61 



probably used in the manufacture of proteins. In the leaves, 

 as elsewhere in the plant, the various foods are digested and 

 thus are prepared to be carried by the tubular cells (sieve 

 tubes) in the veins and down through similar tubular cells in 

 the fibrous bark. (Fig. 16.) 



71. Excretion of the by-products of food manufacture. 

 We showed in 35 that during the process of carbohydrate 

 manufacture oxygen is set free, and this gas is given off 

 through the stomata into the air. Water, too, is a by-prod- 

 uct of food manufacture and assimilation. In the manu- 

 facture of proteins mineral matters are necessary, and these 

 are carried up the stem dissolved in the soil-water. Since, 

 however, the soil-water is such a dilute solution, great quan- 

 tities of this liquid must be supplied ; hence, much more water 

 is taken in than is needed for food manufacture or making of 

 protoplasm. This excess of water is given off in large quan- 

 tity by the leaves of green plants (see 69). 



The amount of water thus excreted by leaves is regulated 

 more or less by the action of the guard-cells that surround each 

 stoma. When the plant is well supplied with water, the stoma 

 remains wide open. If, on the other hand, the leaves lack a 

 sufficiency of water, these guard-cells close in upon the stoma, 

 and so prevent undue loss of moisture. In plants having 

 leaves in a horizontal position, the stomata are mostly located 

 on the lower surface, the upper surface, which is more exposed 

 to the sunlight, being covered with a continuous layer of 

 epidermal cells. Were the epidermis altogether absent from 

 leaves, the mesophyll cells would soon lose so much water 

 that they would die. 



72. Storage of foods in plants. The foods that are 

 manufactured in the chlorophyll-bearing cells may be car- 

 ried, as we have seen, to other parts of the plant, there to be 



