CHAPTER II. — FOOD OF PLANTS. 227 



acids set free or formed by the activities of the plant. Sodium, 

 though present in all plants, and very abundant in some, has not 

 been ascertained to have any very important use. It has been 

 found, however, that it may, to a very limited extent, replace 

 potassium in some plants. Silicon, which occurs in the form 

 of silica, is widely distributed plant constituent, and in some 

 cases is very abundant, as in the Diatoms, Equisetums, and many 

 Grasses, but it appears to have but little physiological importance. 

 Its chief service seems to be mechanical, affording strength or 

 protection to the organ which secretes it. 



Among the accidental constituents of the ash of plants oacur 

 Alumnium, Manganese, Fluorine, Bromine, Iodine, Lithium, 

 Barium, Strontium, Copper, Cobalt, Nickel, Tin, Zinc, and sev- 

 eral others, but most of them, except the first five, are of rare 

 occurrence, and, when present, exist in very small quantities. 



Food of Plants. — The young plant when it begins to germi- 

 nate from the seed, is still practically dependent on the food- 

 stores laid up for it by the parent plant. It is incapable, that is, 

 of deriving its sustenance directly from the soil. Its cells, besides 

 containing protoplasm, are heavily charged with nourishing mat- 

 ters, such as starch, sugar, oil and reserve proteids which the 

 protoplasm makes use of for the purposes of growth. It may also, 

 as we have seen, have an outside supply laid up for it in the form 

 of endosperm or perisperm, which serves the same purpose. 

 When the seed is placed in favorable conditions, as when lodged 

 in moist, warm soil, the dormant protoplasm of the embryo 

 becomes active, water is greedily absorbed by it from the out- 

 side, the stores of reserve-materials are rapidly changed by the 

 acid of ferments present, into soluble forms, as starch and oils 

 into dextrin and sugar, and these are applied to the formation of 

 new cells. As the plantlet increases in size, sending its radicle 

 into the ground, and its plumule, and perhaps also its cotyledons, 

 into the air, its food-stores diminish pari passu and are finally 

 exhausted, and the plant now becomes entirely dependent on the 

 soil and air for its sustenance. In the meantime, it has developed 

 rootlets and numerous root-hairs as absorbent organs, and ex- 

 panded to the air a few green leaves which are to assimilate the 

 absorbed materials. It is evident that its food must now be of a 

 different nature. It must, of course, take in all the elements 



