CIRCULATION OF THE SAP. 107 



other sap. The material which forms the larger 

 portion of the plant structure is not taken from 

 the soil and "elaborated" or fitted for use in 

 the leaves, but it comes directly from the air, and 

 is changed by means of the living cells of the leaf 

 into material capable of promoting the further 

 growth of the plant. Various salts dissolved in the 

 water absorbed from the soil are also essential to 

 the growth and health of the plant, but they form 

 a very small portion of the bulk of the living plant. 

 The downward transfer of the carbon compounds 

 derived from the air occurs as follows: This 

 material, like that derived from the soil, is all in a 

 soluble condition. When any matter which is either 

 soluble, or in solution, is placed in a vessel contain- 

 ing a fluid in which it may be dissolved, it becomes in 

 the course of time uniformly diffused through that 

 fluid, whether any motion exists in the fluid or not. 

 Thus, if a lump of sugar be placed, ever so carefully, 

 in one edge of a dish of water, the whole contents 

 of the dish will in time become equally sweetened. 

 In just the same manner the fluid within the living 

 plant tends continually to become uniform in char- 

 acter throughout. The sap in the lower part of 

 the plant being less dense than that above, con- 

 tinually absorbs the denser material from the leaves, 

 and as this material is constantly withdrawn from 

 the sap in the process of growth, the demand for 



