368 HOW CROPS GKOW. 



The carbhydrates, albuminoids, etc., that are organ- 

 ized in the ioliage, are not only transformed into the 

 solid tissues of the leaf, but descend and diffuse to every 

 active organ of the plant. 



The plant has, within certain limits, a power of select- 

 ting its food. The sea- weed, as has been remarked, 

 contains more potash than soda, although the latter is 

 30 times more abundant than the former in the water of 

 the ocean. Vegetation cannot, however, entirely shut 

 oiit either excess of nutritive matters or bodies that are 

 of no use or even poisonous to it. 



The functions of the Atmosphere are essentially the 

 same towards plants, whether growing under the con- 

 ditions of water-culture or under those of agriculture. 



The Soil, on the other hand, has offices which are pe- 

 culiar to itself. We have seen that the roots of a plant 

 have the power to decompose salts, e. g., potassium 

 nitrate and ammonium chloride (p. 184), in order to 

 appropriate one of their ingredients, the other being 

 rejected. In water-culture, the experimenter must have 

 a care to remove the substance which would thus accu- 

 mulate to the detriment of the plant. In agriculture, 

 the soil, by virtue of its chemical and physical qualities, 

 commonly renders such rejected matters comparatively 

 insoluble, and therefore innocuous. 



The Atmosphere is nearly invariable in its composi- 

 tion at all times and over all parts of the earth's surface. 

 Its power of directly feeding crops has, therefore, a nat- 

 ural limit, which cannot be increased by art. 



The Soil, on the other hand, is very variable in com- 

 position and quality, and may be enriched and improved, 

 or deteriorated and exhausted. 



From the Atmosphere the crop can derive no appreci- 

 able quantity of those elements that are found in its 

 Ash. 



In the Boil, however, from the waste of both plants 



