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.
The idea held by the earlier botanists, that the tips of all roots consisted of spongy masses' of tissue, by means of which plants were enabled to soak up their food from the soil, has, with the aid of the microscope, been entirely discarded. The term spongiole, which was given to these theoretical bodies, is a land-mark of "departed ignorance, and furnishes a striking contrast with the known structure (viz.: the root-cap), with which the tip of every growing root is covered.
With the leading botanists of to-day a root is considered to be an outgrowth protected by a cap. This definition seems very short, but if anything further is added, there comes with it a number of exceptions.
The name which this covering to the root-tip has received is in itself very descriptive, as it is truly a cap, consisting of a number of layers of quite dense cells surrounding the extremity. These root-caps vary in size in different species of plants; sometimes they are so small that only with the high powers of the microscope can they be seen, while on the other hand they may be readily observed with the naked eye.. It is now generally understood that a root does not elongate throughout its whole length, as is known to be the case in the young and growing stem, but its growth in length takes place at a single point a very short distance from the extremity. The stem usually grows in the open air, with plenty of room on all sides, while the root generally penetrates the much denser substance of the soil, where elongation would prove fatal to the delicate root hairs which are thrown out from them on all sides for the absorption of nourishment. Growth in length by elongation of the whole root might also cause twisting and breaking of the roots themselves, and a general disturbance of the soil.
The "growing point" of a root is situated just back of the root-cap, and forms the dividing line between the root and its cap. The position of this point of growth, as related to that of the cap, suggests the use of this latter structure. All young formative tissue is very delicate, and easily destroyed by any external influences. Should this layer of thin-walled, newly formed cells be situated at the very extremity, then, as growth proceeded, these cells of great delicacy would come in direct contact with the rough and sharp edges of the grains of the soil, and soon be broken down and destroyed. It is plain that the important role which the root-cap plays in the vegetable economy is that of protection to the tender growing within, and which it always so completely surrounds. As these outer cells of the cap are worn away, there must be a source of supplies for the protective department, and this is found in the formation of new layers of cells on the inside of the cap, and from the protected tissue.
The growing point, situated near the tip of every root, then, has a double office to perform: the laying down of new tissue for the growth of the root, and the meeting of the losses by wear and tear of its protecting cap.
It may be said, then, that it is not the root-tip which does the work of absorption. Neither do plants "sponge" for a living, but take in their food in a different way, which will be left until another time to describe.