CHAPTER V 



ABSORPTION OF MATERIALS IN GENERAL 



§i. Materials Absorbed by Plants. — We have seen in the preceding chapter 

 that only a few inorganic materials are needed in the construction of the plant 

 body. These essential substances are carbon dioxide, water, and certain salts 

 containing the elements N, S, P, K, Ca, Mg, and Fe, these salts being dissolved 

 in the soil water. From these substances [including the ten essential elements, 

 C, H, O, N, S, P, K, Ca, Mg, and Fe] various kinds of organic compounds 

 are built up by the green plants. Atmospheric oxygen is also absorbed by plants. 

 Absorption of free oxygen does not generally result in an increase in dry weight, 

 however, but is generally accompanied by the elimination of water and carbon 

 dioxide, and thus results in a loss of plant material. Some of the organic 

 compounds thus undergo oxidation through the respiratory process, which 

 will be discussed later. 



Some of the materials that enter the plant are commonly met with in the 

 gaseous form (carbon dioxide and oxygen), others are generally encountered 

 as solids (the salts of the soil, including nitrogen compounds), but they all enter 

 plant cells as substances dissolved in water. In entering, they must all pass 

 through the cell wall, as well as the outer layer of the protoplasm. The 

 mechanics of the absorption of materials by plant cells is thus based upon the 

 laws of controlling the migration of substances dissolved in other substances." 



§2. Diffusion of Gases.— If two gases are separated by a membrane per- 

 meable to them they pass through the spectum and mix. Whether there is a 

 septum between them or not, this mixing process is termed diffusion. Two 

 cases may be differentiated here. The first case refers to septa in which the 

 gases are not dissolved (e.g., a dry porous clay plate). The other case relates 

 to septa in which the gases are dissolved (e.g., moist animal bladder). 6 The 



° Of course the oxygen of the air and of the soil and the carbon dioxide of the air cannot 

 enter plant cells without being first dissolved in water; if not dissolved at a greater distance they 

 go into solution in the water of the cell, which extends to the exterior surface of each exposed 

 cell wall, these walls being impregnated with water of imbibition. The distinctions between 

 solids, liquids and gases have nothing to do, primarily, with the kind of matter considered, but 

 only with its state, which generally depends upon temperature. The author's presentation is 

 here departed from to a certain extent, to avoid his apparent implication that gases enter 

 plant cells in a manner different from that by which substances that are usually solid or liquid 

 make their entrance. — Ed. 



b The term dialysis refers to the process of separating two dissolved substances by letting 

 one diffuse through a septum that is impermeable to the other — a common laboratory opera- 

 tion — and follows the same principles, whether the diffusing substance is commonly met with in 

 the gas, liquid, or solid form. The word osmosis, frequently encountered in connection with 

 the diffusion of substances through membranes, should be dropped, for it does not add to 



104 



