This section is from the book "The Gardener's Monthly And Horticulturist V18", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
That the largest portion of the liquid used by the growing plant makes its entrance through the roots, from the soil, is a well-established fact; but those parts which are the most active in the absorption of this food material in solution were for a long time not so clearly understood.
By careful experiments and microscopic investigation, it is found that the extreme tips of young roots are about the only portion which take little or no part in this work. A short distance back from the growing points, on nearly all growing roots, may be seen with the aid of a microscope a large number of minute, slender bodies extending out in all directions from the surface of the root. These thread-like structures are not unaptly called root hairs, and consist of sac-like protuberances,as outgrowths from the epidermis or surface cells of the root. With the naked eye they are not easily seen, but their presence may be inferred from the manner in which they cling to the particles of the soil when a young root is lifted carefully from the earth in which it was growing. This power which they have of fixing themselves to the grains of earth is very great; so that when a plant is taken violently from the soil, a large part of these delicate hairs are broken from the roots and retain their attachment to the soil. As the root grows along in the earth new hairs are produced while those behind perish as the root becomes woody, and a dense, non-absorbing, protecting epidermis is formed; so that the active life of a single hair is of short duration.
The office of these hairs must have already suggested itself to the reader. By means of these prolongations the greater part of the absorption takes place, though the newly formed surface cells are also active. But the surface which they expose is small in comparison with that of the hairs. It is hard to conceive of a more thorough and economical means of exposing an absorbing surface, at the same time keeping in view their method of apical growth, required strength, and a means of rapid transfer of its liquid through a tube to the root to which it is attached.
In a poor soil roots run rapidly in all directions, and are often very long; so, with the hairs, they are put out quickly, take in what nourishment they find and soon die. While on the other hand, in a rich soil the roots are not required to be long, and the hairs are of greater duration. Here is a saving from the use of manure and other fertilizers which, because below ground and out of sight, is apt to be overlooked.
A consideration of the root hairs involves a question about which there has been much theorizing and speculation, viz: - how do these roof and root hairs take in the liquid from the soil?
Putting aside these various notions, perhaps it will not be amiss to state very briefly the view now generally held, though still thought by some investigators not to explain every point. It is a well-known fact that solid, porous bodies have the power of taking up liquids to a greater or less extent, according to their nature and surrounding circumstances. A dry cloth hung so that one corner will dip into water will in a short time become saturated. This is sometimes called capillary attraction, and has a part in the root absorption. From an extended study and knowledge of the properties of liquids the law of diffusion has been established, viz: - when two or more misible liquids of different degrees of density are placed in contact, interchange will take place until, when the diffusion is complete, the whole liquid will be homogeneous. Instances of partial diffusion are too familiar to warrant space for illustrations. This property of liquids will account for the movement of the absorbed sap to any part of the same cell - from the tip of the hair to the basal portion. But there is another kind of diffusion, first nicely pointed out by Dutrochet and afterwards largely experimented upon by Graham, which has received the name of osmose, or membrane diffusion.
When liquids differing in density are separated by a thin membrane, as a bladder, diffusion takes place through this septum with a rapidity depending on the nature of the liquids and sep-parating membrane, the greater flow being towards the denser fluid. This intercepting membrane often greatly accelerates the diffusion, but just how it acts, and the chemical and other changes which take place in it, or on its' surface, are not well known; still the fact is very striking when, with the proper adjustment of the apparatus and materials, liquids can be made to rise through a tube to the height of many feet. The cell wall of a root hair is such a membrane, separating the denser liquid within the cell from the weaker one without, and this membrane is a living, growing one, and may for that reason be much more effective for osmotic action.
It is difficult to state what portion of the work is done by each of these forces, but combined, they seem to answer the questions concerning the flow of liquids through the plant, as well as the initial step, the entrance of the solutions into the root and hairs. Where these liquids flow after passing into the plant can not he considered here. Suffice it to say, a growing plant is always in a state of unstable equilibrium, with materials in solution continually changing place.
The amount of absorption by the root hairs is often very great, as careful estimates have shown. When rapid evaporation is going on from the leaves, a demand for fluid from below is created which must soon reach the hairs, and they make good the loss.
From the function, position and delicate structure of the root hair at least one important practical conclusion can be drawn - that of the importance of their preservation, when plants are undergoing transplanting, potting or other like change, thus often saving the life of the whole plant.
There are many other plant hairs besides those which grow from the superficial cells of roots, and they may furnish the subject for a few remarks in a future number of the Monthly.