ELEMENTARY CONSTITUENTS OF THE FOOD OF PLANTS. lOl 
phyll 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 pro- 
duction 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 saturating the cell-walls. If the chemical processes within the plant 
are supposed to be at rest, an equilibrium of diffusion will tend to become established 
between the external and internal fluid according to the prevailing conditions. But 
the chemical processes in the interior are continually disturbing this equilibrium, and 
molecules of the salts which are being decomposed are continually streaming from 
without to the places in the interior where they are to be used. If 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 equi- 
librium is being continually disturbed by the separation of the phosphoric acid from 
the calcium, that is, by the chemical change. If the calcium phosphate remained 
as such within the plant, the movement would cease so soon as the equilibrium of 
diffusion 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 these compounds are decomposed ; and that finally the relative quantities of 
the substances in question within the plant need not bear any resemblance to those of 
the substances present in 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 ; again, species 
of Fucus 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 com- 
position of their ash. 
The processes are more complicated 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 
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 
the cell-sap of the root by means of the water saturating the cell-walls of the root- 
hairs. In this manner it is possible for the root to absorb the water of the soil ; as 
this water enters at the points of contact, the equilibrium of the strata of water that 
