CELL-CONTENTS AND FORMS OF CELLS. 249 



and carbon dioxide, the source of the carbon entering into the 

 carbon compounds formed by plants. 



The elements obtained by plants from the soil exist in com- 

 bination with other elements and must be in the form of solution 

 to be absorbed. The soil consists largely of mineral substances, 

 together with certain organic products (humus). The water held 

 in the soil not only acts as a medium for carrying the soluble 

 constituents in the soil to the plant, but is itself an important food 

 product, being the source of the hydrogen used by plants, as also 

 of assimilable oxygen. Among the mineral constituents of the 

 soil that are useful to plants are ammonium salts and nitrates, 

 sulphates, phosphates, chlorides, silicates, and carbonates. When 

 plants are collected and subjected to a temperature of about 110 

 C. the water is driven off, and then if heat sufficient to incinerate 

 the material is applied the organic matter is driven off in the 

 form of gases, leaving the mineral constituents in the form of 

 ash, as calcium, magnesium, iron, potassium, sodium, and a few 

 other elements. 



FORMATION OF LEAFMOLD. When the leaves of a tree fall to 

 the ground they begin to decay and ultimately they are dis- 

 integrated, and their substances become incorporated with the 

 other elements of the soil. The same thing happens with the 

 leaves, stems, and roots of herbaceous plants. Such organic 

 matter is one of the chief sources of food for plants, and its 

 presence in the soil is therefore of fundamental importance in the 

 maintenance of the vegetable mantle of the earth. Coville (Jour. 

 Wash. Acad. Sci., 1913, p. 79) determined the rate in decomposi- 

 tion of leaves and used silver maple, sugar maple, red oak, and 

 Virginia pine. These were exposed to the weather in barrels 

 and in concrete pits. In one experiment a mass of trodden silver 

 maple leaves two feet in depth, with an initial acidity of 0.92 

 normal, was reduced in a single year to a three-inch layer of 

 black mold containing only a few fragments of leaf skeletons 

 and giving an alkaline reaction. Sugar maple leaves have shown 

 a slower rate of decomposition than those of silver maple, while 

 red oak leaves still showed an acidity of o.oio normal after three 

 years' exposure, and leaves of Virginia pine an acidity of 0.055 

 normal under the same conditions. During the decomposition of 



