AGRICULTTIRAL BOTANY. 627 



a new coefficient of humidity be employed in which the water content is taken 



^ ^. - a.T. ,- X i • iu ^ water content . „, „ 



as a function of the humus content, i. e., the ratio r 1 — r i^ "s^Q -ts an 



humus content 



index of the relative soil humidity. The water content is determined by allow- 

 ing samples of the topsoil and subsoil to dry out at about 15° C. until the weight 

 is constant, the loss in weight being the required water content. After subject- 

 ing tlie samples to a red heat the resulting further loss in weight is recorded 

 as humus content. These values are expressed in terms of 100 parts of air-dried 

 soil. The coefficient was submitted to a wide range of tests under variable con- 

 ditions, including different kinds of soils and plants, seasonal variations, and 

 variations in the water content. In most cases results nearly identical for both 

 topsoil and subsoil were obtained which seemed to conform on the whole to the 

 conditions under which they were obtained. 



It is concluded, however, that if the coefficient of soil humidity is to be of serv- 

 ice in ecology it will only be by a study of definite habitats. The mean value 

 of the coefficient of soil humidity is the most important constant to be ascer- 

 tained for every association. This will first make necessary a determination of 

 the supermaximum, maximum, minimum, and. subminimum values of the coeffi- 

 cient for each such association, the subminimum representing the unavailable 

 water. 



Osmotic pressure and related forces as environmental factors, B. E. Lrv- 

 iN( STON (Plant World, 16 {1913), No. 6, pp. i6o-i76).— Summarizing this con- 

 tribution the author states that " the environmental force opposed to water 

 intake may be manifest in any one of 4 forms, all of which are effective in the 

 same way . . , osmotic pressure, the force of imbibition or capillarity, the force 

 of crystallization of water, and the force of evaporation . . . The relative rates 

 of inward and outward movement of water which determine the water con- 

 tent of cells, tissues, and organisms are directly determined by the magnitude 

 of tliese forces on the one hand and of internal osmotic pressure or protoplasmic 

 imbibition on the other." 



Some quantitative researches on the permeability of plant cells, W. J. V. 

 OsTERHouT (Plant Worlds 16 (1913), No. 5, pp. 129-lU)-—^s the result of 

 experiments briefly discussed, the following claims are made: 



The experiments indicate the untenability of the view that the outer layer of 

 the cell protoplasm is composed of lipoid substances, since such a view requires 

 that only salts soluble in lipoid should be able to penetrate. It is considered 

 as proved that the antagonistic action of one salt on another is due to the fact 

 that one salt hinders the other from entering the cell. The usual methods of 

 determining the osmotic pres-sure of cells by means of salts are deemed faulty, 

 true and false plasmolysis being often so confused as to lead to serious errors. 

 It is thought possible by means of electrolytes to cause rapid and very consider- 

 able changes (either increase or decrease) in permeability, these changes being 

 within wide limits completely reversible and entirely devoid of injurious effects. 

 The plasma membrane, it is stated, is readily altered by a variety of substances 

 in a way which indicates that it is protein rather than lipoid. The permeability 

 of the plasma membrane is apparently variable and depends on the nature of 

 the substance with which the membrane is in contact, consequently the internal 

 membranes of the cell (of the nuclei, vacuoles, plastids, etc.) may have a 

 Permeability different from that of the outermost plasma membrane. The elec- 

 trical method employed is considered to be well adapted to studying the progres- 

 sive effects of toxic substances on a plant in its natural environment. The per- 

 meability of a tissue is regarded as n delicate and accurate index of its vitality, 

 affording a means of selecting sound tissue for experimentation. 



