624 CHEMICAL PROCESSES IN THE PLANT. 



The same principles hold good also for the absorption of combinations of food- 

 material from without into the absorbing organ. It has already been shown in the 

 previous paragraph how the decompositon of carbon dioxide in the light in a cell 

 containing chlorophyll induces new quantities of the dioxide at once to enter this 

 cell, whether the gas be at the time dissolved in water or present in the atmosphere. 

 If no carbon dioxide were decomposed in the cell, its contents would become saturated 

 with the gas in proportion to the pressure and the temperature, and every cause for 

 further motion would be removed. But the decomposition is constantly providing more 

 space for the entrance of fresh molecules of carbon dioxide ; and this gas, although 

 present in such small quantities in the atmosphere, collects here and supplies the material 

 for the production of compact masses of carbon-compounds. 



A water-plant acts in the same manner on the salts dissolved in the surrounding 

 water. The external water and the internal cell-sap are in continuous connection 

 through the fluid imbibed in the cell-walls. If the chemical processes within the 

 plant are supposed to be at rest, an equihbrium of diffusion will tend to become 

 established between the external and internal fluid according to the prevailing condi- 

 tions. But the chemical processes in the interior are continually disturbing this 

 equilibrium, the molecules of the salt in question continually streaming from without 

 to the places in the interior where they are to be used. If the molecules of calcium 

 phosphate are even very sparingly distributed through the surrounding water, a dense 

 accumulation will gradually arise in the plant, not of calcium phosphate, but of some 

 other compounds of phosphoric acid and of calcium, because the molecular equilibrium 

 is being continually disturbed by the separation of the phosphoric acid from the 

 calcium, that is, by the chemical process. If the calcium phosphate remained as 

 such within the plant, the movement would cease so soon as the equilibrium of 

 difl'usion was established. It will be at once clear from a consideration of these 

 facts that the accumulation of certain substances in the interior of plants depends in 

 the first place on whether the compound of them which is present in the surrounding 

 water is decomposed in the plant; that moreover the constituents of the different 

 compounds must accumulate in the plant in different quantities according to the 

 extent to which they are needed; and that finally the quantitative composition of 

 the substances in question within the plant usually bears no resemblance to that of 

 the surrounding water. Substances which are present in the water in the form of 

 extremely dilute solutions occur in the plant in great quantities; while others which 

 are abundant in the water are much less so in the plant. Thus, for instance, marine 

 plants take up a much larger quantity of potassium and a smaller quantity of sodium 

 than corresponds to the composition of sea- water; species of Fucus again collect 

 considerable quantities of iodine which is present in sea-water only in extremely small 

 quantities. Since moreover different plants decompose the same compounds with 

 different degrees of rapidity, it is obvious that different plants which draw their food- 

 materials from the same water must exhibit an entirely different composition of their 

 ash. 



The processes are more compHcated when a land-plant has to take up the saline 

 compounds of its food-material from the soil which contains but little water. By 

 far the greater number of land-plants thrive in soil which usually contains a quantity 

 of water much below its full capacity of absorption, its pores being almost entirely 

 filled with air. The small quantity of water present adheres completely to the minute 

 particles of soil, and for this reason does not flow away ; and this adherent water 

 often covers the surface of the particles of earth in the form of a fine stratum. The 

 roots can only absorb this water when they are in the closest contact with the particles 

 of soil ; hence plants freshly planted wither even in moderately moist ground until a 

 sufficiently large number of particles of earth become attached by means of new root- 

 hairs to the newly formed rootlets. At these points of intimate connection between 

 the root-hairs and the soil the adhering water of the latter is directly continuous with 



