LEAVES 195 



municating with the external atmosphere through the sto- 

 mata, which are able to regulate the freedom of this com- 

 munication. Through this mechanism, the carbon dioxide 

 of the external atmosphere diffuses into the internal atmos- 

 phere of the leaf and passes into the working cells, while 

 the oxygen that passes from the cells into the internal at- 

 mosphere diffuses into the external atmosphere. 



(3) Veins. In the cross-section of the leaf there appear 

 sections of the veins of various sizes, embedded in the meso- 

 phyll at varying intervals (Fig. 157). It has been stated 

 that the veins bring to the mesophyll cells water that has 

 come from the soil. It will be seen that the water-con- 

 ducting part (xylem) of these vascular strands is towards 

 the upper surface of the leaf, the natural position when it 

 is remembered that the strand arises from the stem cylinder, 

 where the xylem is on the inside, and turns outward into 

 the horizontal leaf. 



121. Photosynthesis. The manufacture of carbohydrate 

 food by green plants was outlined in Chapter III (p. 31) in 

 connection with Algae, so that it need not be repeated here. 

 However, it should be remembered that the process de- 

 scribed there is not only carried on by the leaf, but that the 

 leaf is a particularly efficient machine for photosynthesis 

 in plants exposed freely to the air. Water is carried in from 

 the soil, carbon dioxide diffuses from the external atmosphere 

 to the internal atmosphere through the stomata, the chloro- 

 plasts in the mesophyll cells lay hold of these materials with 

 energy obtained from light and manufacture a carbohydrate, 

 the oxygen waste diffuses from the internal atmosphere to 

 the external atmosphere, and the whole living machine is 

 kept from drying out by the epidermis with its layer of 

 cuticle. 



122. Transpiration. Although water is being evaporated 

 constantly from the surface of a living plant exposed to the 

 air, foliage leaves are especially exposed to this evaporation 



