558 STATE BOARD OF AGRICULTURE. 



cell, aud its environment. Consequently llie permeability of the mem- 

 brane will be constantly clian<>inj>'. It is better to accept the view of 

 selective permeability, thon<;h we cannot exj)lain it in its intricate 

 phases, than to assume that the membrane is quite impermeable to many 

 of the solutes and that when these do penetrate, the membrane must 

 "leak." 



The concentration of tl»e cell sap is rarely or never llie same as that of 

 the solution outside the i)lant, or in the conductinj^- tissues of the plant. 

 There is therefore established a movement of water into the cell, and a 

 l)roduction of a hydrostatic pressure equivalent to the osmotic pressure 

 of the dissolved substances. This pressure ])ushes the i)r()toplasm out- 

 ward and keei)s the mendjrane tense and rijj;id. Osmotic pressure there- 

 fore has a definite function. It maintains and regulates turgor. A 

 knowledge of solutions is necessary to a ])ro])er understanding of this 

 ])henonienon. 



Much information has been collected to show that the molecules of sol- 

 utes obey the well known laws of gases. A substance in solution in water 

 acts essentially as a gas. These particles are not so free to move as in 

 the gaseous state because of the friction between them and the molecules 

 of water and thus great pressures are necessary to move them through 

 the Avater or to separate them. This applicability of the laws of gases to 

 solutes has made evident the proper basis of comparison between solu- 

 tions of different compounds. So, now, solutions for comparison are 

 equimolecular ones, and are not made on the percentage basis. We must 

 take this fact into consideration in studying the influence of difTerent salt 

 solutions or nutrient solutions on the physiology of plants. The problems 

 of balanced solution and antagonistic action have depended almost en- 

 tirely for their explanation on a knowledge of physical chemistry. 



Conductivity measurements have helped immensely in the study of the 

 influence of concentration on the absorption and excretion of solutes by 

 the roots of plants while growing in dilute culture solutions. There is a 

 certain concentration for each salt or mixture of salts at which the roots 

 excrete aud absorb electrolytes at the same rate. At certain concentra- 

 tions on either side, one function overbalances the other. Thus a knowl- 

 edge of the culture concentration which is best fitted for the development 

 of the plant can be obtained through conductivity studies. The con- 

 ductivity method is the easiest and most accurate, since the resistance 

 to the passage of an electric current is a function of the number of ions 

 present in the soluti<m, and the number of ions is in turn the index of the 

 concentration. This is true only in dilute solutions where complete dis- 

 sociation has taken olace. 



The presence ot colloids, tliat are of constant occurrence in tlie soil, 

 gives rise, in the soil solalion. to a nhenomenon called ad.sorption. Altho 

 a subject containing little that is definitely known, it is nevertheless of 

 much importance. A more thorough study from the vantage point of our 

 present knowledge of physical chemistry should clear up much of the 

 haze that envelops this subject. Physical adsorption as it is often called 

 to distinguish it from absorption and chemical combination is defined as 

 a concentration of one component on the intei-face between two phases. 

 In dilute solution the solute is very likely adsorbed in ionic condition and 

 if the assumption is true, that the adsorbed ions are then inoperative, 

 the conductivity of the solution would be a measure of the amount of 



