268 PLANT PHYSIOLOGY 



Plants are greatly influenced by the calcium content of the 

 soil, especially by calcium carbonate. In their relationship to 

 these salts, plants may be divided into two large groups: those 

 that have a high requirement for lime, or calciphiles; and those 

 that avoid limestone soils, or calciphobes. These two groups 

 are linked by intermediate plants. They can be sharply sepa- 

 rated only by considering their extreme representatives. These 

 differences, however, are of a purely physiological character. 

 They cannot be distinguished by any anatomical or morphological 

 peculiarities. Not many of the higher plants are definitely cal- 

 ciphobes. Good examples of such plants are the lupines, the 

 chestnut (Castanea vesca), sphagnum moss, and several other 

 plants of sphagnum bogs. 



As calcium represents one of the indispensable elements for 

 the development of higher plants, it seems at first sight incom- 

 prehensible how there can be plants that do not grow on cal- 

 careous soils. Yet the ash of such lime-avoiding plants contains 

 normal amounts of CaO. This becomes understandable, how- 

 ever, if it is remembered that an increased or decreased lime 

 content results in chemical and physical changes of the soil. 

 Soils that contain much lime have a neutral or slightly alkaline 

 reaction advantageous to bacterial activity and the accumulation 

 of humus, which is favorable to plant growth; while in acid soil, 

 fungi develop, and an acid humus accumulates, which is injuri- 

 ous to many plants. The increase in calcium carbonate also 

 improves the physical properties of soils, such as their porosity 

 and their heat absorption. It is interesting to note that in water 

 cultures all plants, calciphilous as well as calciphobous, require 

 the addition of a sufficient amount of calcium ions for their 

 normal development. With sufficient acidification of the solu- 

 tion, even the most calciphobous plants grow perfectly well in a 

 high concentration of calcium salts. Conversely, upon alkali- 

 55Ution of the soil solution even a small surplus of calcium is 

 injurious. Thus the relation of plants to lime is closely con- 

 nected with their relation to the concentration of hydrogen ions 

 in the soil. 



62. Effect on Plants of the Hydrogen-ion Concentration of 

 the Soil. — Of all the cations affecting the development of plants, 

 H+ ions are the most active. One of the principal properties 

 of acids and alkalies, as well as water, is the faculty of dissociating 



