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PART III. — VEGETABLE PHYSIOLOGY. 



mentioned above as essential constituents, and that it may thrive 

 to the best advantage, it must have also those elements which 

 were designated as of minor importance. But it no longer takes 

 them in such complex forms as those in which they were stored in 

 the seed. On the other hand, none of them, save oxygen, can the 

 plant utilize in the elementary form ; and even its consumption 

 belongs largely, if not wholly, to the respiratory rather than to 

 the assimilative process. They are absorbed in the form of inor- 

 ganic compounds. A considerable portion, even of the oxygen, 

 is obtained from water and from various salts absorbed by the 

 plant. The carbon is derived from carbon dioxide ; the hydro- 

 gen, mainly, at least, from water ; most of the nitrogen from 

 ammonia, ammonium salts and the nitrates ; the sulphur from the 

 sulphates ; the phosphorus from the phosphates ; chlorine from 

 the chlorides ; potassium from the phosphate, chloride, sulphate 

 and probably also the silicate ; sodium, mainly from the chloride, 

 and calcium, magnesium and iron from the sulphates, carbonates, 

 nitrates and phosphates of these elements. 



The carbon dioxide, made use of by the plant as food, is 

 absorbed from the air. Oxygen is taken by land plants partly 

 from the air, and partly from solution in the water that perme- 

 ates the soil. The mineral salts required by plants exist in 

 minute quantities in the dust of the atmosphere, yet in proportion 

 large enough to supply the needs of epiphytes ; but they occur 

 in still greater abundance in most soils, which is the source 

 whence the great majority of plants obtain their supplies. Besides 

 these inorganic salts there are in most soils various decomposing 

 organic matters, which many plants are able to appropriate ; but 

 that these are not really essential to plant life, is shown not only 

 by the fact that Cactus plants and House-leeks grow on bare rocks, 

 or in arid sands, but also by experiments like the following : If 

 the root of a germinating Bean be placed in solution containing, 

 in 1,000 parts of water, about two parts of the following com- 

 pounds, potassium nitrate, iron phosphate and calcium sulphate, 

 and its leaves be exposed to the sunlight and air, care of course 

 being* taken that a suitable temperature be maintained, it will 

 grow nearly, if not quite, as well as if planted in the soil. 



Absorption of Food. Carbon dioxide, or at least that 

 portion of it which is utilized as food, reaches the interior of the 



