Fig. 32.—Experiment to
prove that green leaves,
exposed to sunlight,
give off water.
The use of water to the leaves.—It is common knowledge that if a twig is allowed to become dry its leaves hang limply and wither; but if the twig is allowed to dip into water the leaves will keep fresh and crisp for a considerable time. This necessity for supplying the twig with water seems to indicate that leaves give off water, and that this is so may be proved by a few simple experiments. Two tumblers may be arranged as in [Fig. 32]: separated by a card through which passes the end of a leafy twig. The end of the twig dips into water in the lower tumbler. In order to prevent water vapour from passing from the lower tumbler to the upper, the chinks between the twig and the card are sealed with paraffin wax.
When this arrangement is placed in the sunlight, a dew soon collects on the inside of the inverted upper tumbler. This water must have been given off in the form of vapour from the leaves. That the loss of water from leaves is due rather to the light than to the heat of the sunshine may be shown by keeping leafy twigs in the dark. The leaves keep fresh much longer than when placed in the light, even if they are kept in as warm a place.
The pores of the leaf-surface.—An ordinary leaf remains fresh much longer if its lower surface is smeared with vaseline. The explanation of this lies in the fact that the waterproof skin of a leaf is perforated by a multitude of little pores, especially on the lower surface. In most leaves, indeed, the pores are confined to the lower surface. Smearing the surface with vaseline blocks up these pores and thus prevents the escape of water vapour from the interior of the leaf.
These little mouths (known as stomata)[6] open in the light and close in the dark. During the daytime, therefore, the air (containing its small proportion of carbon dioxide) has free access to the interior of the leaf through the stomata, and, on the other hand, any water which the leaf does not require can escape in the form of vapour. A leaf requires water not only because all its mineral food ([p. 29]) is brought to it dissolved in water, but also because water as well as carbon dioxide is required for the manufacture of the starch and other plant-foods.
How plant-food is distributed.—The water which comes up from the roots is distributed to the various parts of the leaf through the veins. These are therefore not only supports, which stretch out the soft leaf-stuff to the light and air, but they also form a very complete network of irrigating channels or water pipes. Further, the starch and other foods which a leaf makes are drained off into the stem through other minute channels which are bound up with the water pipes. The starch, for example, is changed into a kind of sugar which dissolves in water and drains away. From the stem, the food solutions are distributed to all the parts where growth is taking place.
EXERCISES ON CHAPTER III.
1. Make drawings of as many cases as possible of economy of leaf surface.