326 TEXTBOOK OF BOTANY 



contact with soil particles that the salt ions adsorbed on their surfaces 

 may move from one to the other without actually being in solution. 

 Thus the entrance of a salt into roots is influenced not only by the con- 

 centration of the salt and the permeability of the protoplasm, but also 

 by the electrical charges on its ions and those on the surfaces of soil 

 particles and of root cells. The physical and chemical processes involved 

 in the absorption of salts other than simple diffusion are so complex that 

 we shall omit further consideration of them. 



It may be noted, however, that any salt in the soil surrounding the 

 roots may enter the plant. Neither the kinds nor the proportions of salts 

 found in plants bear any necessary relation to "what the plant needs." 

 After the ions of a salt have entered a plant they may remain in solution, 

 become a part of organic molecules, form salts with organic acids and 

 bases, accumulate in crystalline form, or be adsorbed on the surfaces of 

 the colloidal particles in the protoplasm and cell walls. The ease with 

 which some of the ions of a salt may be removed from the protoplasm 

 without killing the cells may be demonstrated by placing leaves of 

 elodea in water at 90° F. for an hour or two. Some of the calcium in the 

 protoplasm is set free and forms beautiful crystals of calcium oxalate 

 with the oxalic acid that was already free in the vacuoles of the cells. 



Relation of inorganic salts to plant development. Most people know 

 that the growth of a plant is often improved after certain salts have 

 been added to the soil, and that the practice of applying fertilizers con- 

 taining salts of nitrogen, phosphorus, and potassium to fields and gardens 

 would be discontinued if it were not profitable. On some kinds of soils 

 it is profitable to apply salts containing certain other elements. 



Fig. 138 illustrates the relative development of a tomato plant when 

 nitrogen or some one of several mineral elements is not present in the 

 salts added to the sand in which the roots grow. It is customary to refer 

 to these plants as having grown in a sand culture; that is, the pots were 

 filled with pure quartz sand, and then a culture solution, made by placing 

 a known amount of certain chemically pure salts in distilled water, was 

 added from time to time. Perhaps no chemical element is absolutely lack- 

 ing in these experiments. Many kinds of chemical elements were in the 

 compounds in the seed from which each plant grew, and it is impossible 

 to obtain absolutely pure sand, water, or salts. It is quite evident, how- 

 ever, that any one of several elements may be so deficient that the plant 

 grows but little and fails to complete its entire life cycle. 



The characteristic differences in growth and appearance of a plant. 



