The cells in the leaf and in other parts of the plant have the powerto change the sugar and combine it with other substances contained in the sap, thus forming more complex chemical compounds. These contain nitrogen and sulphur, besides the elements of the sugar. Such compounds are called proteins. They are essential to the formation of plant protoplasm and are very important as animal foods.
A FOREST OF PINES.
From the sap of these, turpentine and resin are made.
The digested and soluble substances which are prepared by the leaves are transported to other parts of the plant, where they are combined by the protoplasm of the living cell with other substances contained in the cell sap. Thus the protoplasm itself is able to increase and form new cells as well as other substances, such as woody tissue and oils and resins. In forming these substances the plant uses oxygen just as animals do. If air is kept from the roots of certain plants, as was seen in Experiment 94, the plants cannot live.
These food substances which plants make by using the energy supplied by the sun are the bases of all plant and animal life. The sun"s energy stored up in the green leaf is the source of all plant and animal energy. If it were not for the leaf manufactory run by the sun"s power, life, as we know it, would cease. Even white plants, like the mushroom, must live on the food manufactured by the chlorophyll of the green plants.
Experiment 103. -Procure a small thrifty plant growing in a flower pot. Take two straight- edged pieces of cardboard sufficiently large to cover the top of the flower pot and notch the centers of the edges so that they can be slipped over the stem of the plant and thus entirely cover the top of the flower pot. Fasten the edges of the cardboard together by pasting on a strip of paper. The top of the pot will now be entirely covered by the cardboard but the stem of the plant will extend up through the notches of theFig. 97.
edges. Cover the plant with a bell jar. No moisture can get into the bell jar from the soil in the pot as it is entirely covered. Set the plant thus arranged in a warm sunny place. Moisture will collect on the inside of the bell jar. This must have been given out by the plant leaves.
Since all the processes of forming new material by the plant require large amounts of water, it can readily be seen why water is so essential to plant development. The water from which the food materials haveA SUNFLOWER PLANT.
been taken is thrown off by the leaves, as seen in Experiment 103. The amount of water thus thrown off by plants is very great. A single sunflower plant about six feet tall gives from its leaves about a quart of water in a day, and an acre of lawn in dry hot weather gives off probably six tons of water every twenty-four hours.
If the water passes out of a plant too rapidly so that there is not enough left toprovide for the making and transporting of the food, the work of the plant cannot be carried on, and the plant dies. It is on account of this that many plants are especially prepared to retain their water supply. In almost all plants the stomata, or little pores in the leaf through which the water passes out, close up when too much water is being lost.
In some plants, like the corn, when the root cannot supply sufficient moisture, the leaves curl up and thus present less surface for evaporation. In trees like the eucalyptus the leaves hang vertically when the sun gets too bright and present their edges to the sun"s rays. Some leaves, like the sage, are especially prepared to conserve their moisture by having their surfaces covered with hairs. Others have a waxy covering, asthe cabbage and the rubber tree. In some
EUCALYPTUS LEAVES.
plants the leaves are very small and have few pores, as the greasewood of the desert, and some have done away with leaves altogether, as the cactus. It is because the roots cannot supply sufficient moisture where the ground freezes in the winter that trees having large leaves shed them, and only trees like the pine whose needle-like, waxy leaves give off almost no moisture can retain theirs.
95.Flowers. -The stem not only bears leaves but, in the higher kinds of plants, it bears flowers. The function of the flower is to pro- duce seeds and provide for the continued existence of its kind. If the flower of a buttercup, quince, cassia, or geranium is examined, it will be found to be made up of four distinct kinds of structures.
Around the outside is a cluster of greenish leaves. This is called the calyx. Within the calyx is the corolla, a cluster of leaves which in many plants are colored. Within the corollaare a number of parts consisting of a rather slender stalk with an enlargedtip. This tip is called the anther, and the
stalk and anther together, the stamen.
FLOWER, SHOWING COROLLA,
STAMEN AND PISTIL.
In the center of the flower are the pistils. At the top of a pistil is generally a somewhat enlarged portion, the stigma, which is sticky or rough; and at the bottom there is an enlarged hollow portion, theseed-bearing part, called the ovary. These two parts are connected by the stalk-like style. The stamens and pistils are the essential parts of the flower, the calyx and corolla being simply for protection or assistance. All flowers do not have these fourparts, but every flower has either
PINK GENTIAN.
Showing the anthers which are covered with pollen.
stamen or pistils or both.
The anther produces a large number of little granular bodies, called pollen grains, each of which consists of a free cell containing protoplasm. When the pollen grains are ripe, the anther opens and exposes them. If a pollen grain of the right kind falls upon a stigma, it grows and sends down a tiny tube through the style into the ovary, where a little protoplasmic cell, called the egg cell, has been produced. The essential parts of these two different kinds of protoplasms unite and a new cell is formed.