252 PLANT PHYSIOLOGY 



in connection with the intake of dissolved substances into plant 

 cells in general (see Art. 8 in Chap. I). A great significance in 

 this process may be attributed to the specific permeability of 

 these cells to definite ions, to Donnan's equilibrium connected 

 with the presence of colloids in the cell, and to the cells' active 

 absorption of ions by the expenditure of the energy obtained by 

 respiration. 



Dining recent years, the attention of investigators has been 

 especially attracted to the connection existing between absorp- 

 tion of ions and respiration of roots. The investigations of 

 Hoagland, Petri, Lundegardh, Sabinin, and others have definitely 

 established that respiration of the roots is one of the essential 

 conditions for the absorption of salts; and this explains the 

 favorable influence, observed long ago, of the aeration of soil 

 and the bubbling of air through water cultures for the growth of 

 plants. 



The nature of this connection between respiration and absorp- 

 tion of ions is not clear as yet, and various authors think it 

 occurs in different ways. The main role evidently belongs to 

 the function of H+ and HCOs" ions, formed in respiration and 

 translocated to the surface layers of the protoplasm, to exchange 

 with the colons and anions present in the surrounding solution. 

 The intensity of respiration is determined to a considerable 

 degree by the accumulation of respiratory materials, the carbo- 

 hydrates. This accumulation in turn depends on the intensity 

 of photosjmthesis, which is the chief source of carbohydrates in 

 the plant organism. Investigations establish quite definitely 

 a close connection between the accumulation of organic substance 

 and mineral salts in the plant. This connection was frequently 

 noted by previous investigators, but its nature remained obscure. 



The mutual interdependence of the main vital processes in 

 the plant, viz., photosynthesis, respiration, and absorption of 

 mineral salts, may serve as a good example of the importance of 

 studying the various processes occurring in the plant, not 

 isolated from the rest of the functions and from the life of 

 the organism as a whole, but in close mutual connection and 

 interdependence. 



59. Mineral Nutrition of Plants Grown in Soils. — In soil, the 

 mineral nutrition is much more complicated than in an artificial 

 culture; for the plant encounters here a great variety of com- 



