92 Introduction to Botany. 



stomata are exceedingly small, being only about T f^ milli- 

 meter in diameter (see Fig. 35), but this small size is com- 

 pensated by the great number of the stomata, of which 

 there may be, in different plants roughly estimated, from 

 fifty to seven hundred in each square millimeter of epi- 

 dermis. While the water is thus evaporated, the salts, 

 brought into the plant in solution from the soil, are left be- 

 hind in a more concentrated solution, from which they are 

 abstracted as needed, in the building up of new compounds. 



Oxygen is also given off by the leaf, and, for the most 

 part, doubtless through the stomata; the source of this 

 oxygen is the carbon dioxide which has been worked over 

 by the chloroplasts, for they use only the carbon, and set 

 the oxygen free. Thus, while the daylight is strong 

 enough to furnish the necessary energy, there is a con- 

 stant stream of carbon dioxide into, and of oxygen out of, 

 the leaf ; so that while the plant is forming its food, and 

 its food is likewise our food, it is making the air purer 

 for breathing. The process of starch formation by leaves 

 under the influence of the sunlight is called photosynthesis 

 a putting together by light. 



The diagram shown in Fig. 42 illustrates the mutual 

 relations of root, stem, and leaf in the absorption, manu- 

 facture, and translocation of materials needed by plants. 



70. Respiration. After the sun goes down, photosyn- 

 thesis ceases, and another process is found to be going on 

 in the leaves ; that is, the leaves are taking in oxygen and 

 giving off carbon dioxide ; this they were also doing dur- 

 ing the daytime, but the oxygen evolved by photosynthesis 

 predominates so much that the consumption of oxygen at 

 that time is obscured. We see, then, that the leaves are 

 taking in carbon dioxide and giving off oxygen during the 

 daytime, and are taking in oxygen and giving off carbon 



