18

NATURE OF SPONGELETS.

any fluid or gaseous matter that may be presented to them. On this account they are usually called spongelets.

In the roots of ordinary exogens, when the tissue is very young, the spongelet (Fig. I. a) consists of very lax tender

cellular tissue, resting upon a blunt cone of woody matter, composed principally of woody tubes, and connected with the alburnum of the stem (Fig. I. b); it is, therefore, placed in the most favourable position possible for communicating to the general system of circulation the fluids taken up by its highly absorbent tissue. In some roots a cap exists, called pileorhize, which guards as it were the spongelet beneath it, or forms part of the spongelet.

a

It is the opinion of most vegetable physiologists, that the absorbing or feeding powers of roots are conducted principally at these points; and that the general surface of the root possesses little or no power of the kind. And, indeed, it seems highly probable that this is so, when we consider how thick is the bark of the root, through which fluids would have to pass before they reach the alburnum.

But although there can be no doubt that the spongelets act as absorbents with more force than any other part of the root, yet it is equally certain that the whole surface of young roots also possesses an absorbing property, only in a more limited degree. It is not until their tissue is solidified that roots

THEIR ABSORBING FORCE.

19

become incapable of passing fluid through their sides; and when very young and soft, there is probably but little difference between their action and that of the spongelets themselves; for it is to be remembered that the latter are not special organs, but are only the very youngest part of the root.

The absorbent power of the spongelets must be much greater than would have been supposed, if we consider that it is almost entirely through their action that the enormous waste of fluid, which takes place in plants by perspiration, is made good; and hence their importance to plants, and the danger of destroying them, become manifest.

Roots being furnished with the power of perpetually adding new living matter to their points, are thus enabled to pierce the solid earth in which they grow, to insinuate themselves between the most minute crevices, and to pass on from place to place as fast as the food in contact with them is consumed. So that plants, although not locomotive like animals, do perpetually shift their mouths in search of fresh pasturage, although their bodies remain stationary.

Many examples of this might be adduced. The following are, however, sufficient. In a Garden at Turnham Green, a Populus monilifera (Canadian Poplar) was found to have sent a root thirty feet horizontally, including its dip beneath the foundations of a wall, and then to have passed into an old well to the depth of eighteen feet, having then broken up into a mass of fibres so finely divided as to resemble yarn.

In another case, a root of the Deciduous Cypress was found by the author, eleven feet long, which had passed nearly to that length without division in search of water.

Mr. Tyso, a Florist at Wallingford, mentions the case of a Mignonette plant which had penetrated through several courses of bricks, and descended far into a wine cellar. Over the cellar, which was outside the dwelling-house, was a brick pavement, between the joints of which Mignonette seed had been sown from year to year. At the extreme end a small portion of soil was allowed, and here a plant or two grew more vigorously than the rest, though not so luxuriantly as is often found in a common border. The roots of these plants had penetrated through eighteen inches of brickwork, and some of them were hanging inside the arched roof, nourished by the damp atmosphere only. A few, more favourably situated, were attached to the end wall of the cellar, and had descended five feet five inches down the wall into the decaying sawdust of the wine-bin. Others were beautifully spread over the wall, with a

EXCESSIVE DEVELOPEMENT OF ROOTS.

thousand ramifying rootlets, bespangled with minute crystal-like damp drops, and extending over a space of five feet in width. It was difficult to trace the brittle roots that had penetrated the sawdust, but he measured some upwards of seven feet below the surface of the brickwork on which the plants were growing. It is this peculiarity which renders it so difficult to keep drains or wells in working order where roots have access. A well six feet wide has been known to be filled with roots by a common Laurel bush. Turnips and Mangel Wurzel, as well as mere weeds, have great power in this way. Patrick Neill mentions an instance of a plant of Ragwort (Senecio Jacobea) which had insinuated the point of its roots into a drain, and had then extended them so much as to fill the drain completely for about twenty feet. And thus it is seen that it is by the point that roots extend, with an indefinite power of branching, and that the finest thread once introduced into a drain-pipe will rapidly become the origin of most extensive mischief, provided the plant is perennial. A still more remarkable case is mentioned in the Gardeners' Chronicle for 1849, of a line of pot-pipes from forty to fifty feet long, socketed and cemented, and thought to be perfectly closed, having become so choked by roots as to be unserviceable in fifteen years. In the side of one of the pipes there had been one mere chink; and through that chink some tree had insinuated the point of some root. Once inserted the point lengthened and divided, and lengthened and divided over and over again, till at last the drain-pipe was filled by an entangled mass of fibres which had pressed so firmly against each other as to form in some places a tolerably perfect mould of the cavity.

Roots lengthen, as already stated, not by extension, but by perpetual additions of very soft cellular matter to their points. That matter is in fact in the beginning mere mucilage, capable of organisation. A small portion of this mucilage finds itself in contact with a minute

INCREASE OF ROOTS IN DIAMETER.

21

cleft; conditions, the real nature of which is unknown, cause the mucilage to press against the cleft; the mucilage is introduced, it organises, solidifies, and the point of a root is established in the cleft. The point forms more mucilage in advance; that also solidifies, and a further lodgment is made; and thus the growth goes on, through all the sinuosities of the narrow passage that it traverses. In the annexed figure, the space from a to b represents the part where the root passed through the pipe, which must have been nearly two and a half inches thick; the root, there, was as thin as paper, and had followed every bend in the crack. As soon as it reached the inside of the pipe (at b) it swelled, acquired its usual cylindrical form, and thence proceeded to develop and branch in the manner already described. The thin connecting plate was sufficient to maintain the vitality of the roots for many years.

The only known exceptions to the rule that roots do not lengthen by a general distension of their tissue, occur in parts growing in air or water, which are non-resisting media, or in certain endogenous trees, whose roots lengthen to such a degree as to hoist the trunk up into the air, off the ground with which it at first was level.

It is not, however, merely in length that the root increases; if such were the case, all roots would be mere threads. They also augment in diameter, simultaneously with the stem, and under the influence of exactly the same causes. Neither is it by an embryo alone that roots are formed. A plant, once in a state of growth, has the power of producing roots from various parts, especially from leaves and stems.

The well-known emission of roots by the stems of the common Laurel is a phenomenon due, as it seems, to the death of the lower part of the stem, the live part of which is thus compelled to organise its descending sap in the form of roots. Vines in damp hot-houses, with their roots in a cold border, habitually exhibit the same tendency. And as a further illustration, one published by Mr. W. B. Booth may be introduced. This was the case of a Spanish Chestnut between ninety and one hundred years old, and of considerable size, cut down in 1849. With the exception of its foliage, which always had a yellowish, sickly tinge, there was scarcely anything else about it that indicated decay. Its trunk seemed perfectly sound, and the young shoots it annually made, appeared to be pretty strong and healthy. No sooner, however, had the workmen commenced cutting, than it was discovered that for ten feet high, as much as two-thirds of the bark round the trunk was dead and reduced to a mere shell. On removing this thin covering, the sap

22

22

FORMATION OF ROOTS BY THE STEM.

wood was found to have become a mass of decayed vegetable matter, through which a complete net-work of roots passed to the ground, and extended themselves for a considerable distance from the main stem. Some of these roots were about the size of an ordinary walking-stick.

Fig. III.-Spanish Chestnut which had thrown out roots under the bark ten feet
above the ground.

On tracing them to their source, they were observed to spring from the edge of the healthy portion of the tree, immediately above the part that had been injured and gone to decay; and as only a few of the larger ones reached the ground, the whole of the nourishment conveyed by the others to the tree, must have been derived from the gradual decomposition of its own sap-wood. A still more remarkable case is