PLANTS. 65 



Food from the Soil. The only substances that plants 

 obtain from the atmosphere are carbon, v small quanti- 

 ties of oxygen, and probably some nitrogen in the case of 

 certain particular plants. All other nutritive substances, 

 generally including nitrogen, must come from the soil. 



A plant obtains these nourishing substances from the 

 soil through the moisture in which they have been dis- 

 solved. The moisture of the soil passes through the 

 membranous covering of the root fibers, and thence up- 

 ward into the plant, forming its sap or juice. Sub- 

 stances dissolved in this juice are thus able to find their 

 way to all parts of the plant. In order to understand the 

 rise and flow of sap, and the distribution of nutritive sub- 

 stances, it is necessary to consider three principles of 

 natural philosophy called diffusion, osmose, and capillary 

 attraction. 



1. Diffusion. Diffusion is the term applied to the ten- 

 dency of different liquids, and solids dissolved in liquids, 

 to become thoroughly mixed when placed together. If 

 alcohol and water are placed together in the same vessel, 

 they quickly mingle, so as to form a uniform mixture. 

 If a handful of salt is thrown into a pail of water, it is 

 quickly dissolved, and evenly distributed, so that all 

 parts are equally salted. 



The same is true if several substances, as salt, sugar, 

 and alum, are dissolved in the same water. Each is 

 equally distributed as if no other were present. These 

 facts are due to the attraction of molecules of alcohol for 

 those of water, and the attraction of molecules of water 

 for those of salt, sugar, etc. The fact that all the mole- 

 cules of water have equal attraction for salt causes the 

 salt to be equally distributed between them. If, in any 

 way, some of the salt could be withdrawn from a portion 



WINS. AG. 6 



