482 



Agricultural Experiment Station, Ithaca, N. Y. 



Just why so much water is required by the growing plant and 

 how it obtains this supply is not usually understood. It has 



been the subject of considerable re- 

 search and even now presents inter- 

 esting problems for further study. 

 The roots of the plant are its feeders 

 and all of the water ordinarily used 

 by it passes in through these chan- 

 nels. The particles of soil hold a 

 film of water in firm contact. The 

 roots and rootlets of the plant, in 

 burrowing through the soil, come 

 into intimate relation with these soil 

 particles (Fig. 140). The finer the 

 soil, the closer the relation estab 

 lished between it and the roots. The 

 The roots are thus surrounded by a 

 thin film of water, a portion of 

 which they are able to absorb. The 

 water passes up through the tissues 

 of the plant, carrying with it soluble 

 plant food which is conveyed to the 

 manufacturing or elaborating or- 

 gans, the leaves. There, in the pres- 

 ence of sunlight, the fixation of carbon from the air takes place 

 and by means of the movement of the sap the now organized 

 material is carried to all growing parts of the structure. That 

 part of the water no longer recpiired i)asses off through the 

 breathing pores of the leaf, called stomata. As evaporation 

 is a cooling process, there is no doubt that this loss of water 

 has an important influence in lowering the temperature of foliage 

 and in promoting the fixation of carbon. 



As already stated, the plant roots can absorb food on\y in the 

 soluble form, and the passage of a large quantity of water 

 through their tissues is necessary to furnish the su])])ly of min- 

 eral elements required by growth. Not only is a large quantity 

 of moisture demanded for the direct use of the i)lant, but its 

 presence in the soil is necessary in order that the plant food may 



140 — A corn plant showing the inti- 

 mate relation between the roots and 

 the soil particles. From life. 



