Popular Science Monthly/Volume 59/September 1901/Plants as Water-Carriers
|PLANTS AS WATER-CARRIERS.|
By Professor BYRON D. HALSTED,
A GIANT redwood, the monarch of the California forests, stands with its stem-tip three hundred and fifty feet above the soiL From the surface of the millions of tender delicate leaves near the top of the tree there are exhaled many gallons, perhaps barrels, of water daily. The force required to make good this loss is, of course, equal to that needed to raise the water through the three hundred feet or more of vertical space. It is no wonder that the thoughtful person will pause as he contemplates this exhibition of force. It makes no noise; work is being done, but it is not easy to see how.
Let us begin with the soil, as that is the source of the water supply of plants, and briefly consider its constitution, texture and relations to the problem of water-carrying. In other words, does soil carry water and, if so, in what way is it conveyed? Soil is rock that has been broken into small pieces in one way or other, a refinement of rock, so to speak, whether by frost, moving water or chemical action. For our purpose soils, having many degrees of fineness, may be classified into coarse, medium and fine. Coarse soil may be compared to masses of cannon balls touching each other but with large spaces between them, while the medium soil is similar to peas in piles, and the fine soil is like clover seed. The chief difference is in the amount of surface exposed by the particles which go to make up a definite portion of soil.
The next point for us to consider is the capacity of soil for holding moisture. Thrust the hand into a dish of water and upon removal it will be wet, except any portion that has been coated with oil or similar substance. In short, water will leave its own mass and adhere to the surface of the hand. If the hand held a quantity of clean earth the latter would likewise become wet. The amount of water that the soil will hold depends upon the surface exposure of its particles. As this is an exceedingly important point, permit me, at the risk of dealing largely in dry figures but for explanation and proof, to draw upon some results given by Professor King in his charming book 'The Soil.' With columns of sand ten feet long, the one with the grains averaging in diameter 186/10,000 inch, after percolating for 111 days, contained 3.77 per cent, of water; the 73/10,000-inch grains retained 4.92 per cent.; the 61/10,000-inch grains, 5.76 per cent, and the 45/10,000-inch grains, 7.57 per cent. In other words, the smaller the size of the grains and the pores the greater the amount of water retained. With soils of still smaller particles the water-holding power would be correspondingly greater.
If the soils are placed in long upright glass tubes and water is added at the bottom, it will rise through the soil to the top. This phenomenon of capillarity is best illustrated with tubes having bores of varying diameter. The tables given us on this subject are to the effect that, when an inch tube is plunged into a vessel of water, the height of the water column in the tube above the general level is.054 inch; for a 1/10-inch bore.545 inch, 1/100-inch bore 5.456 inches and for a 1/1,000-inch bore, 54.56 inches. While the actual surface pull of the smallest tube is much less than that of the largest, it is through a vastly greater distance and by a multiplication of the number of such minute tubes in a given space that the greater lifting is brought about.
The soil itself, consisting of minute particles, admits of the capillary action; for the pores, although not straight, extend in irregular lines and permit the surface tension that is evident in fine tubes. This lifting power of minute passageways is abundantly illustrated in the everyday operations of crop-growing, and the skilful tiller makes abundant use of it or checks it as best suits his purpose. If a dark soil contains an abundance of light alkaline salt, it is possible that it may have a white crust form upon the surface during a drought to be carried back upon the falling of a substantial rain, and this rise and fall may be repeated indefinitely as happens on some of our alkaline lands, where the precipitation is light and vegetation scant.
It has been shown that the soil, on account of its porosity, is able to lift water through considerable distances, simply through the greater pull of a solid for the liquid than the liquid has for its own particles. The hand is wet by the water; a towel hung high with barely one corner dipping into the basin may become wet throughout, and, by evaporation, the dish may be pumped dry.
Into this complex physical porous mixture, to the component particles of which a liquid adheres with such force as to be present when even the air is dry, the plants establish themselves by means of their tiny rootlets and the much more minute root hairs which, insinuating themselves between the microscopic pebbles, become misshapen and contorted beyond all recognition of the simple vegetable cells out of which they have grown. The movements of the water in the soil, whether to the right or left, up or down, are governed, as has been shown, by the law of surface attraction. When we come to the plant cell, the whole physical basis is changed, and, among other things, we are brought face to face with membranes of extreme thinness and delicacy, and, more than all, with the living protoplasmic film.
The two ends of the microscopic aqueous canals, in plants of the ordinary sort, are the active imbibing root-hairs, above mentioned, and the exposed surfaces of the aerial parts, chief of which are the leaves. The lower terminal is buried beneath the soil from which it receives its supply, and consists of a sac with a very thin and elastic wall, lined with a delicate film of living protoplasmic substance. We may imagine one of these cells many hundred times enlarged, its contents consisting of a thin syrup, but slightly more dense than the liquid in which it is immersed. As time passes, there is a flow inward of the less dense liquid and an increase of the wall tension of the cell. This tension might be observed by pricking the wall, when from the pinhole the liquid would spurt for some distance. The same pressure might aid in the passage of the liquid from the sac to the one next adjoining, and in that way a flow would be set up from the less to the more dense cell contents.
A homely and common illustration of this osmosis or membrane diffusion is seen in the action of sugar upon ripe strawberries, the sugar taking the thin juice from the cells and making a syrup that finally surrounds the berries. Place dried prunes in a dish of warm water, and a similar exchange is demonstrated, but in this case the flow is into the dry cell contents, and the prunes finally become plump. There has been a transportation of liquid in both these instances, and it has been from one cell to another through the whole necklace, so to say, of many beads, from the surface to the innermost cell or vice versa, as the case may be.
Let us ascend a tall tree, figuratively, and study microscopically the upper terminals of the lines of water-carriers. Here we find the leaves in great numbers, presenting, possibly, acres of actual visible surface to tile drying influence of the almost constantly changing air. But if we note the exceedingly porous structure of a leaf, how one cell touches its fellows at but few points and the bulk of the space is intercellular, the actual surface exposed to the atmosphere is a hundred times more than the naked eye reveals. As with the soil terminal, so here the end of the transportation line divide up into a million parts. In the former, each is for the reception of liquid; in the latter, they are all places of unloading. The drying air sweeps over them, and something of their contents is vaporized and is gone from the plant. But this evaporation increases the density of the cell contents and were there no reserve the tissues would wither, dry up and become dead, as is the case when a branch is cut off or grass is mown in the meadow.
If we apply the law illustrated in the dried prunes, it will be seen that each surface cell in the loose pulp of the leaf is dependent upon the next below it, and that, in honoring the draft upon it, is making a physical call upon another, and so the line is established, like men passing buckets at a fire, or tossing melons in the loading of a schooner for a northern market.
The whole story of water-carrying is not ended with the above. One of the most delicate of all plant mechanisms is that which is associated with the transportation of its liquids. The leaves and green surfaces generally are closely studded with minute structures, 100,000 or more to a square inch, that open or close as the emergencies of the case demand. They are vitalized and exceedingly sensitive valves, usually constructed of two crescent-shaped cells set in the skin and highly charged with protoplasm. These organs are influenced by sunlight and darkness, by heat and cold; in fact, their functioning calls forth the admiration of any careful student of the subject. The two guard cells are so hung that they become turgid when the leaf is well filled with water, and thus enlarge the opening to its full capacity for the passage of vapor-laden gases. As soon as these guard cells lose much water, they become less plump, and this brings about the closing of the pore. They are, therefore, valves of safety, and, as the other portions of the leaf are covered with a cuticle more or less impervious to gases, it is seen that the stomates are the organs that regulate the evaporation stream.
That the amount of water carried is very great scarcely needs to be emphasized. Note the rapidity with which grass wilts when cut for hay or the leaves upon a branch that has received any injury. If a melon vine with twelve leaves will carry a liter of water in a single day, as it has been known to do, what must be the vastness of the lift in a forest of a thousand acres upon a dry day when the leaves are fresh and most active!
That it needs to be great is seen from the requirements of the plant. The soil water is weak in all salts that a plant must acquire, and to take them in concentrated form would be as poison. The whole plan, therefore, is to carry large quantities of a dilute solution, and afterwards bring it to the required strength. In the evaporation there is a cooling obtained that may possibly save the plant from destruction.
We thus far have seen that an ordinary plant has its slender, delicate, insinuating root-hairs closely applied to the soil particles from which they imbibe the adhering moisture. It has further been shown that the opposite terminal of the waterways has also a vast number of delicate living cells exposed, not dangerously, to the drying action of the atmosphere. Between these two extremities is the body of the tree, the main roots and branches, and it is for us to determine through what parts the upward flow takes place. This admits of demonstration by the removal of certain portions and observing the effects. That it does not take place through the central or heart wood is to be expected, for the cells here are often all filled up with lignin and coloring matter, and the way is blocked; the canal is filled with débris, so to say, and has become disused. Again, the old central wood frequently decays until there is only the outer ring of the later-formed wood remaining with the bark that covers it. That the bark is not the water-carrier may be shown by removing a ring of it and thus breaking the connection without interrupting the upward flow. That it does pass through the young wood may be shown by cutting this portion without harming materially the bark or the heart wood, when the leaves quickly wither and the tree may die. In short, the sap-wood is well named, as through it the soil-water mounts upward from the roots to the leaves.
In many plants, however, there is no well-developed ring of wood. Either the stem is too young to have one or its construction such that it never appears. However, the same kind of tissue is somewhere to be found in the stem, usually in strands or portions of tough threads, as in the corn-stalk, and through these the crude sap is transported. Some of these succulent stems are so transparent that they admit of experiments which demonstrate both the path and the rate of the upward flow. For example, a balsam stem may be cut and, while fresh, plunged into a harmless colored liquid, as that of some aniline dye. It is found that the woody bundles are the first to take the stain and that it mounts upward with a rate that is an index of the flow of sap and may be some feet in a single hour. Another test for the rate is found in the use of a harmless salt, easily detected in extremely minute quantities by the spectroscope. Let it be lithium nitrate, for example, and its rise discovered by making sections of the stem at different distances and burning small fragments.
But having determined the place of entrance, line of ascent and point of departure of the aqueous stream, it by no means follows that all the forces have been named that bring about the transfer. That living plants carry water and make it one of the chief labors of all their active days is beyond question, but physicists and physiologists, chemists and biologists are as one concerning the mystery that here exists. A grape-vine stump bleeding in early spring is a stumbling block for them all, and they fall back upon 'root pressure,' a term more convenient for covering much ignorance than for service as a full, well-rounded explanation of the phenomena in question. Membrane diffusion will account for much, capillary attraction helps considerably, and the differences of gas pressure within and without the cells, as in the tapped sugar maple in early spring, count for something; but back of all is a vital force that has not been reduced to a physical or chemical basis.