MANUAL OF AGEICULTURE. 261 



tinderstood that plants absorb the liberated ammonia in a direct 

 manner, and nitric acid in the same way ; although in regard to 

 the latter the question is more undecided. These, accordingh-, 

 are the natural sources whence plant life obtains the necessary 

 organic elements of carbon, hydrogen, oxygen, and nitrogen. 

 The inorganic compounds, whence are derived the inorganic ele- 

 ments necessary for the uj^building of plant structure, exist in the 

 soil in, as we have seen, an almost insoluble condition. But rain 

 water holding carbonic acid in solution has, we know, its soluble 

 power over such compounds in the soil vastly increased. But 

 even with this, and the additional fact of so large a quantity of it 

 permeating the soil and plant structure (experiment has shown 

 that for every grain of inorganic matter assimilated by a plant, 

 2000 grains of water have passed through the latter), still plant life 

 could not derive inorganic substance in sufficient quantity, with- 

 out the additional agency of the power stated to be inherent in 

 roots, of contributing by chemical or other means to the solubility 

 of the compounds of this class with which they come into con- 

 tact in the soil. By heat and light these several absorbed com- 

 pounds, when exposed in the leaf or elsewhere, are broken up, 

 and their atoms and molecules rearranged, to form the proximate 

 and other compounds found in plants. 



The several constituents of plant life " all form," says John- 

 ston, " more or less constantly and abundantly a portion of the 

 fixed and solid matter of the plant taken as a whole. They may 

 not be found in any one part of the plant when separated care- 

 fully from the rest ; but in the solid parts of the plant, taken as 

 a whole, they are all and always to be met with. When thus 

 deposited they become, for the most part, dormant, as it were ; 

 and for t;he time cease to perform an active chemical function in 

 the general growth ; though, as vessels or cells, they may still 

 perform a mechanical function. They undergo various chemical 

 changes in the interior, chiefly while circulating or contained in 

 the sap, by which changes they are prepared and fitted for enter- 

 ing, when and where it is necessary, into the composition of the 

 solid or fixed })arts of the plants. Thus the starch of the seed is 

 changed into the soluble dextrin and sugar of the young plant, 

 and then again into the insoluble cellular fibre of the stem or 

 wood as the plant grows ; and finally, into the insoluble starch of 

 the grain as its seed lills and ripens. They each exercise a 

 chemical action more or less distinct, decided, and intelligible 

 upon tlie other elementary bodies, and the compounds of them, 

 which they meet with in the sap of the plant. In regard to some 

 substances, such as potash and soda, the sulphuric and jdiosphoric 

 acids, this last function a])pears to be especially imporLunt. 

 These substances inlluence all the chemical changes which go on 

 in the interior of the plant, and which modify or cause it? growth. 



