PLANT PROPAGATION 111 



referred supply the mineral ingredients needed by plants in the form 

 of soil-water. But soil, to be fertile, must contain a considerable 

 quantity of vegetable mold, the so-called humus, a dark brown or 

 black substance produced by the decay of vegetable matter. This 

 is the reason that florists mix with the dirt in their flower-pots a 

 handful of material obtained from the floor of the forest (see 

 frontispiece), where leaves have fallen and decomposed year after 

 year. 



122. Moisture. If the student has tried the experiment in 

 119, he will have been convinced that the amount of moisture 

 supplied to seeds or plants has a great deal to do with their develop- 

 ment. Soils in very dry or arid regions are deficient in water, 

 and this must be supplied by irrigation. In semiarid regions proper 

 methods of tillage, as we shall see, will do much to keep the soil 

 in a proper condition for plant growth, so far as moisture is con- 

 cerned. Very moist or "heavy" soils, on the other hand, are 

 unfavorable for the growth of most plants, and so the excess of water 

 must be removed by drainage. 



Reviewing some of the facts already learned, we see that a large 

 supply of water must be secured by plants from the'soil, because 



"1. A living plant contains a large proportion of water gen- 

 erally more than 75 per cent of its weight. 



"2. Large quantities of water must pass through the plant in 

 order that the food solution in the soil may be carried to the leaves, 

 and the substances that it contains may be converted into organic 

 matter. This water loss takes place by transpiration from the 

 leaves and growing shoots. 



"Careful and extended experiments in this country and Europe 

 have shown that 300 to 500 tons of water are taken from the soil 

 by the various crops for each ton of dry substance produced." l 



123. Relation of the soil to air. When the interstitial spaces 

 between the particles of soil are not filled with water, or when they 

 are only partly filled, they contain air. The air which circulates in 



1 Bailey's " Cyclopedia of American Agriculture," Vol. I, " Farms," 

 p. 353. " 



