CHAPTER XIII
LEAVES—FUNCTION OR WORK
We have discussed (in Chap. VIII) the work or function
of roots and also (in Chap. X) the function of stems.
We are now ready to complete the view of the main vital
activities of plants by considering the function of the
green parts (leaves and young shoots).
Sources of Food.—The ordinary green plant has but two sources from which to secure food,—the air and the soil. When a plant is thoroughly dried in an oven, the water passes off; this water came from the soil. The remaining part is called the dry substance or dry matter. If the dry matter is burned in an ordinary fire, only the ash remains; this ash came from the soil. The part that passed off as gas in the burning contained the elements that came from the air; it also contained some of those that came from the soil—all those (as nitrogen, hydrogen, chlorine) that are transformed into gases by the heat of a common fire. The part that comes from the soil (the ash) is small in amount, being considerably less than 10 per cent and sometimes less than 1 per cent. Water is the most abundant single constituent or substance of plants. In a corn plant of the roasting-ear stage, about 80 per cent of the substance is water. A fresh turnip is over 90 per cent water. Fresh wood of the apple tree contains about 45 per cent of water.
Carbon.—Carbon enters abundantly into the composition of all plants. Note what happens when a plant is burned
