PHYSIOLOGY OF CORN PLANT 45 



found in large quantities ; but these are probably not essential 

 to growth. 



THE ABSORPTION OF WATER 



35. It has been pointed out in the text that the water 

 absorbed by plants is a dilute solution of all the soluble 

 substances in the soil, the absorption taking place through 

 the vast number of root-hairs, from which the water solu- 

 tion passes into the lateral roots, up the stem, and out 

 into the leaves. The water passes up the fibrous bundles 

 found all through the pith. This can be demonstrated 

 by cutting off a stem near the ground early in the morn- 

 ing, when root-pressure is high. Water will soon exude 

 in small drops wherever the fibro-vascular bundles are cut. 

 During the heat of the day, root-pressure is negative, and 

 no result can be secured. 



THE GIVING OFF OF WATER 



36. Water loss ^ from the plant serves several functions, 

 the most important of which is the concentration of the 

 water solution. By constant evaporation of water the 

 salts taken up in solution are left in the plant, to.be 

 utihzed in its growth. The leaf is so constructed as to 

 facilitate the giving off of quantities of water and at the 

 same time protect the inner tissues. 



The leaf is covered with a strong epidermis, which has, 

 however, an enormous number of stomata. The number 

 in a single corn leaf of average size is estimated at sixteen 

 to twenty milUons. These small openings are connected 



1 Water loss from the plant is of two kinds, namely, transpiration and 

 evaporation. The former is closely associated with assimilation, and the 

 amount of water given off as «■ result of this process is comparatively 

 small. The greatest loss is by simple evaporation, in common with aU 

 objects exposed to dry air. 



