4. What Plants Do With Water and How They Breathe
Some one has said that one day without water would make men liars, in two days they become thieves, and after the third or fourth day they would kill to get water. In the Army Records at Washington is a report of one of our expeditions, which in chasing Indians got lost in a desert, and in which the soldiers fought among themselves for even the most repulsive liquids. It hardly needs these gruesome examples, however, to confirm what everyone who has ever been mildly thirsty knows, that water is an essential for all animals, and that to be without it is to suffer torture. Air of the proper kind is just as important, and because its absence or impurity causes more sudden agony and a quicker death, the need of it is that much more acute. Plants rely even more upon these two essentials of life, and in getting them they behave in ways just as ruthless as do men who are suddenly deprived of either of them.
As we have already seen in “How Plants Get Their Food and Water from the Soil,” the water is the carrier of the food elements from the soil, but water as such does much more for the plant than act as a carrier. Osmotic pressure, a never-ending pump, keeps sending up a steady stream of water to the limits of its power. In everything except trees it seems fairly certain that this pressure is sufficient to drive water into the remotest leaves. It finally reaches these tiny rooms in the leaf about which we read in the account of Leaves as Factories. And just here a very curious thing happens. Each room is, as we have seen, a very busy place, crowded with all the necessary equipment to make sugar, and yet there is still room for water which is just as necessary as the other fittings; in fact so necessary is it that the whole interior of the room is bathed in water. This irrigation system works so well that the walls of the room literally bulge with the pressure of the water in them. If they did not—a condition known as turgor—the plant would at once wilt, and if no new supply came it would wither and die.
But water cannot stay in this condition of pressure and stagnation for even a brief period. That would be as if a leaf were like a toy balloon which, after inflation, had the entrance pinched and so remained inflated. And while we have all along spoken of factories for making sugar, and pressure pumps for forcing up food and water, it must never be forgotten that this marvelously adjusted mechanism is a living thing. Constantly growing, even producing their own means of falling in the autumn, leaves must be thought of as living machines, just as we are still more highly developed machines. In other words the accumulated water in the cells of the leaf must be removed, after it has served its use, and replaced by fresh supplies. The removal is carried on by its evaporation into the halls, or, in the more precise terms of our account of leaves as factories, into the intercellular spaces. It will be recalled that these are connected with the outside air through the pores or stoma. When the air outside is hot and dry it might easily suck out by evaporation all the water vapor in these intercellular spaces and wilting follow at once. This would actually happen if the guard cells, already mentioned, were not constantly on the job. They control the size of the opening just as certainly as a steam valve does, and maintain, with a few exceptions, just the proper amount of water loss not only to maintain turgor, but to see to it that transpiration, as this process is called, goes on rapidly enough to insure fresh supplies of water being sent to the leaf. The opening and closing of the stoma by the guard cells is a nicely balanced operation dependent upon root pressure, turgor, and atmospheric conditions. Guard cells have, because of this, been much studied in spite of the fact of their microscopic size. We now know that they allow greater openings during the night and reduce them during the day. When we reflect that the constant removal of water in the leaf, both as such, and as the only carrier of food supplies from the roots, depends in such large measure upon the functioning of these guard cells, then we come to some realization of their importance to the plant.
They do not always work unaided, for in many places the transpiration, even with their best efforts, would exceed the rise of water in the plant and death must follow if such a condition exists for long. This may be the case in certain bog plants, where, even with their roots in the water, they actually are in danger of drying out because the composition of bog water makes it partially unfit for most plants. And, again, in very open dry or windy places, such as deserts or the mountain tops above timber line, the actual supply of water may be insufficient. Many plants growing in such places have their leaves, particularly the under surfaces of them, clothed with various kinds of hairs. These may be quite velvety or cottony, but in any event, either by their texture or their color, they tend to reduce transpiration. An extreme case is a desert plant from Arizona where the whole leaf surface is covered with an ashy gray velvety coating, which, of course, absorbs less heat than a normal green leaf, and in addition there are much fewer pores through which transpiration could be carried on. In ever so many leaves nature has provided them with a thick coating of hairs in early spring, which they lose later in the summer. Shrubs and herbs, especially those that start earlier than the trees under which they grow, very often may be found with a dense woolly or silky covering in early spring. As the shade becomes denser and the need of the protection less, the wool or silk is shed, sometimes completely. Some of the most conspicuous cases of this are certain kinds of our common shadbush, which in April are covered with a beautiful grayish-white silky coat, but by August are practically the ordinary green color of other leaves. The hairy covering of leaves is well worth observation, as it may hide not a few facts about transpiration and, in some leaves, has had much to do with their preservation from grazing animals. Some, like the common mullein, are never touched, and may be found standing like sentinels in fields otherwise cropped short.
In many leaves there is conflict between those forces that result in the leaf getting the utmost possible exposure to light and those that prevent too rapid transpiration. On the one hand there is the absolute necessity for light, on the other the ever-present danger that the response of leaves to this necessity will result in a transpiration rate too rapid to be held in check by the guard cells. The compromise between these two forces, each pulling in opposite directions, gives to some leaves a series of movements that are among the most interesting things in nature. One of the most marked examples is the common wild lettuce, a weedy plant of our roadsides introduced from Europe. In bright sunlight the leaves are turned so that the edge of the blade faces upward, and the surface is thus protected from the direct rays of the sun, but during cloudy weather or in the shade the leaves turn into the ordinary position of most foliage leaves. It is difficult to avoid the inference that photosynthesis, which, as we have seen is an absolute necessity to the leaf, is in the wild lettuce retarded by transpiration, to avoid the too rapid rate of which the leaf is turned on edge. In this plant the leaf base, as though to be ready for whatever change transpiration or photosynthesis may demand, is so attached to the stem that such changes are made with the least possible delay or wrenching. In one of the many kinds of blue gum trees of Australia all the leaves turn one way in the light, and another in shade or on cloudy days. Ever so many plants have partial movements of their leaves, a good many of which are in response to these opposing demands, one pulling the leaf into the greatest possible light, the other holding it away from that condition. There are other movements of leaves, of parts of the flower, or even of the whole plant that are not so certainly the result of the conflict between light requirements and the necessity of conserving water supply. They will be considered presently.