48 PLANT PHYSIOLOGY AND ECOLOGY 



6 1. Effect of protoplasm upon the absorption of soluble salts. 



The protoplasmic membrane of the cell is not merely a powerful 

 factor in determining the absorption of water. It also exerts 

 an extremely important action upon the amount of soluble mate- 

 rial taken in with the water. The latter is not absorbed as it 

 is found in the soil. The protoplasm has a different degree of 

 attraction for the molecules of water and for those of the dis- 

 solved salts. Moreover, it has a greater affinity for some salts 

 than for others. In consequence, the soil water that enters the 

 plant not only contains a changed amount of soluble salts, but 

 the relative amounts of these are also different from the amounts 

 present in the soils. The rule apparently is that proportionately 

 more water than salt is absorbed when the solution is relatively 

 concentrated, and proportionately more salt than water when 

 the solution is dilute. An analysis of the ash of species growing 

 in the same habitat not only shows that soluble salts exist in 

 the plant in a different proportion from that found in the water, 

 but also that this proportion is different in each species. This 

 power of the protoplasm to take up water and soluble salts with- 

 out strict regard to their proportion is of very great importance 

 to the plant. The plant is enabled to filter out as it were some 

 of the dissolved salt when the concentration of the latter is higher, 

 and it is unnecessary or injurious. On the other hand, the root- 

 hairs are able to absorb proportionately more nutrient material 

 when the solution is dilute, thus avoiding the necessity of absorb- 

 ing an excess of water in order to secure the necessary amount 

 of salts. Furthermore, this property enables the protoplasm to 

 absorb more of the salts that are being used most, and less of 

 the others. Indeed, the property is probably to be ascribed to 

 the difference in the demands of the plant. 



62. Diffusion. The passage of absorbed soil water from the 

 root-hair cell to a neighboring cell, and from this to the other 

 cells of the root, takes place by virtue of osmosis. The absorbed 

 water first passes into the cortical cell, and in return a certain 

 amount of cell sap passes into the cell of the root-hair. This 

 process of diffusion is continued throughout the cortical region 

 until the water taken in by the root-hairs is finally brought to 

 the fibrovascular region. Diffusion, however, is not due to os- 

 mosis alone. The attraction of protoplasm for water, as well as 

 for other substances that it needs, doubtless plays a part. In 



