Popular Science Monthly/Volume 38/November 1890/The Root-Tip
By FREDERICK LEROY SARGENT.
IT is only within recent years that botanists have realized what a wonderful organ the root has at its tip. Text-books which were in use twenty-five years ago give but little more upon the subject than the statement that at the extremity of each rootlet is a minute, sponge-like organ, called the spongiole, by means of which the plant absorbs moisture from the ground. As long ago, however, as 1837, Ohlert showed that if this so-called spongiole be cut off from a young root, and the wound covered with water-proof varnish, absorption takes place quite as well as before the operation; and he expressed the opinion that the true organs of absorption are numerous delicate hairs which form a velvety zone a short distance behind the apex of a rootlet. Later investigators have confirmed Ohlert's conclusions, and have found that the terminal organ, instead of being absorbent like a sponge, is in reality a protective cap, and as impervious to water as cork. (See Fig. 1.)
Just behind this cap, and inclosed by it as a thimble covers the finger-tip, lies that part of the root which is youngest and tenderest, where growth is most vigorous, and from which all the
Fig. 1.—Parts of a Young Root (Pentstemon). (1) Seedling, with earth-particles attached to the root-hairs. (2) The same, showing the root-hairs freed from earth-particles. (3) Root-tip penetrating the soil ( x 10). (4) Root-hairs with earth-particles adhering (x 50). (5) Vertical section of root-tip, showing protective cap and growing point (x 30). (Kerner.)
other tissues of the root are derived. This vegetative point we may consider as the tip proper. (See Fig. 1 (5).) As fast as the surface wears off by contact with the earth, new tissue is added beneath, much the same as one's finger-nail is constantly renewed, and thus the thickness of the cap remains about the same, although continually worn away.
The new tissue which is added to the body of the root soon loses the power of increasing in length, and consequently the elongation of a root is in marked contrast to the elongation of a stem. The latter, to be sure, has, like the root, a small mass of formative tissue at the apex, but the tissue which is formed continues to enlarge for a comparatively long time, and the result is that a young stem grows in length at a nearly uniform rate throughout, while in a rootlet elongation takes place only near the tip. The simple experiment of making a series of equidistant ink-dots along the stem and root of a bean seedling will, as growth proceeds, give a good idea of the difference in manner of growth. It is obvious that were a root to elongate like a stem, the results could hardly fail to be disastrous: for, in the first place, the resistance of the earth would soon cause a strong curvature; and, in the second place, the tender apex would be injured by being thus forced against the earth. As it is, the tip penetrates the earth, not like a nail driven by a force behind, but like a slender, tapering cone whose point insinuates itself between the earth particles and then by growth in thickness wedges them apart. Experiment has shown that a root in its longitudinal growth exerts but very little force; in the bean, for example, there is scarcely force enough to raise a quarter of a pound. The force of transverse growth, on the other hand, is considerable–equal in the bean to the raising of over eight pounds.
It was first demonstrated by Darwin that the elongation of the root takes place in such a way that the apex, instead of going straight forward, bends to all sides in succession and thus describes a somewhat corkscrew-like spiral. This movement he called circumnutation, and found that essentially similar movements (some of which had been before observed) were exhibited by all growing stems and leaves, and not infrequently after growth had ceased. In the case of the root, the movement may be rendered apparent in either of two ways. One method is to take a seedling growing in moist air, and magnify the movement of the root-tip by attaching to the bending portion a very slender filament of glass several inches in length, and then, on a sheet of glass kept perpendicular to the axis of the root, record by inkdots the different points to which the filament is from time to time directed. Upon connecting the dots made at short intervals through a period of several hours, a result is obtained somewhat like that shown in Fig. 2. The other method is to allow the vertical root of a seedling to grow downward against the smoked surface of a piece of glass which is held oblique to the axis. If the conditions are favorable, the tip will be found to rub the surface and leave a serpentine tracing similar to those given in Fig. 3. That the course of the tip had been spiral and not zigzag was shown in Darwin's experiments by alternating regions of greater and less rubbing, and in some cases by transverse ridges of soot. Since these experiments can not be performed with the root imbedded in compact earth, we can not say how far circumnutation may take place in ordinary soil, but undoubtedly the tendency to circumnutate is ever present, and whenever there is favorable opportunity for its exercise the spiral movement must materially assist the tip in making its way along the line of least resistance. The chief importance of this power of movement, however, comes from the way it may be modified, and its force augmented in certain directions by different influences.
Prominent among these influences is that of gravity. A most noticeable fact in the sprouting of seeds is that the root points toward the center of the earth, and the young shoot in the opposite direction, and it has long been known that this tendency to assume the vertical can not be explained as a response to differences in illumination, warmth, or moisture, since the organs behave just the same when seedlings are grown under conditions where these differences are entirely eliminated. Moreover, if a root which has been growing downward be placed in a horizontal position, the region of growth, for a few millimetres behind the tip, will in the course of some hours bend so as to bring the tip into its original vertical position; and as this bending will take place against an appreciable resistance, it follows that the assumption of the new position is not a mere drooping, but is a movement actively performed as if in response to a stimulus.
Fig. 2.Fig. 3.
Fig. 2.—Circtmnutation of Radicle (Brassica) traced on horizontal glass from 9 a. m. January 31st, to 9 p. m. February 2d. Movement much magnified. (From Darwin's Power of Movement in Plants.)
That gravity is the stimulus which evokes this response, was first proved by Knight in 1806. He reasoned that "as gravitation could produce these effects only while the seed remained at rest and in the same position relative to the attraction of the earth, ... its operation would become suspended by constant and rapid change of position of the germinating seed, and it might be counteracted by the agency of centrifugal force." He accordingly attached a number of germinating beans in various positions to the rim of a wheel, and this, placed in a box sufficiently warm and damp, was made to turn in a vertical plane at the rate of one hundred and fifty revolutions a minute. After a few days, the parts of the seedlings were found to be in the position shown in
Fig. 4.—Diagrams illustrating Knight's Experiments. A, wheel rotating horizontally; the plants grow under the combined influence of gravity arid centrifugal force. B, wheel rotating vertically; the direction of growth is determined by centrifugal force alone. (Vines.)
Fig. 4, b. Fig. 4, a, shows the position assumed by seedlings placed under conditions entirely similar, except that the wheel was made to turn horizontally. Since both gravity and centrifugal force were here acting at right angles to each other upon the seedlings, the oblique direction of their axes shows that they were affected by the resultant of the two forces concerned, in just the manner called for by Knight's supposition.
Although gravity is thus seen to be the influence which" induces a downward tendency in roots, it of course does not follow that all the younger parts of a root-system are equally affected. While it is the rule for primary roots, or those first developed, to grow downward, the secondary branches usually tend to assume a direction almost at right angles to the vertical, and so grow outward and a little downward, as if they were but slightly susceptible to the action of gravity; while tertiary branches, and the farther branches to which these give rise, grow in all directions quite independent of gravity. It is plain that as a result of these peculiarities the active parts of the root are distributed in such a manner as to search the surrounding earth more thoroughly than would otherwise be possible.
In case a stone or other obstruction is encountered by any of the branches, the tip is turned aside and follows the contour closely until the edge is reached, when it soon assumes its proper direction. Not infrequently it must happen that some root-eating animal will destroy the end of a young primary root, and so endanger the proper development of the whole system, but experiment has shown that in the event of such injury one of the younger secondary branches changes its direction of growth so as to point directly downward and thus assume the function of the primary root to promote the search for food in the deeper regions.
At first sight it would seem that surely gravity must affect all parts of the growing region of a rootlet in the same manner, since all parts are equally exposed to its influence. In 1871, however, Ciesielski announced that rootlets from which the tip had been carefully removed with a razor lost all sensitiveness to gravity until a new tip had grown, when the behavior became normal. Other investigators failed to obtain the same results; but some years later Darwin repeated Ciesielski's experiments successfully, and confirmed his conclusion that it is the tip alone which is sensitive to gravity, and from this part the stimulus is transmitted to the adjoining region of growth, which bends downward in consequence.
Another influence to which roots are very sensitive is that of moisture. This is strikingly exhibited in an experiment devised by Sachs. Seeds are made to germinate in a layer of moist sawdust, contained in a sieve-like framework, and this suspended obliquely as shown in Fig. 5. The young roots grow directly downward through the loose mass and out through the meshes of the sieve, when, instead of continuing vertically, they bend toward the moisture which comes from the sawdust and keep close to the inclined surface in spite of gravity.
Fig. 5.—Apparatus to illustrate the Mode in which the Influence of Gravity is overcome by the Effect of Greater Moisture on one Side of the Root. (Sachs.) With a view to seeing whether this sensitiveness to moisture was localized like the sensitiveness to gravity, Darwin covered the tips of a number of seedlings with grease, and then subjected them to an excess of moisture on one side. No bending occurred so long as the tips remained covered. This led him to believe that sensitiveness to moisture is confined to the same part which is sensitive to gravity, and later investigators, using improved methods, have confirmed Darwin's conclusion. The lateral branches, being less controlled by gravity than the main axis, are, as might be expected, more responsive to differences in moisture. So delicate is this sensitiveness that the roots oftentimes seem to work almost intelligently in their search for water. Thus elm roots have been found filling up a drain fifty yards from the trunk, and numerous instances of roots growing into wells and choking water-pipes have been reported.
A very common effect of this special sensitiveness is to regulate the distribution of the rootlets in accordance with the water-shed from the leaves. The greater part of our trees shed the rain outward like a dome or spire, so that the region of earth best watered falls directly under what may be called the eaves: it is just here that the tips of the rootlets occur in most profusion. In the case of shrubs and herbs, which are more apt to grow close together, the water-shed is, of course, mostly indefinite, and as a consequence no regularity is apparent in the distribution of the rootlets; but even among herbs quite definite water-shed is not uncommon, and as with trees the effect upon the rootlets is well marked largely in proportion to the isolation of the plants. Certain kinds shed the water outwardly like the trees (Fig. 6, 1), while others have the leaves so disposed as to act like a funnel and carry the water toward the axial root around which the short rootlets are developed (Fig. 6, 2).
It has already been mentioned that the root-tip, when coming against an obstruction, turns aside and thus avoids being pushed against it. This has been taken to indicate that the tip is sensitive to contact as well as to moisture and gravity. To test this supposition, Darwin tried the experiment of affecting one side of the root-tip with a slight but constant mechanical irritant. In
Fig. 6.—(1) Centrifugal Water-shed in Caladium, and (2) Centripetal Water-shed in Rhubarb—showing corresponding distribution of rootlets. (Kerner.)
some cases the irritation was produced by a tiny bit of card attached obliquely to the tip by shellac or gum; shellac by itself was sometimes used, and in other instances the sensitive region was touched with caustic. In nearly every case the tip became bent away from the side irritated (Fig. 7). Occasionally it happened that the region just above the tip became irritated (by displacement of the card or otherwise), and in such cases the end of the root was bent strongly toward the source of irritation. These results seem to warrant the conclusion that the end of the root is not only sensitive to contact, but responds in opposite ways according as the side of the tip or the region just above is affected, and we get an explanation both of the way the tip bends when meeting an obstructing surface, and of the abrupt curve it makes when the edge of the obstruction is reached. It has been urged, however, that these experiments do not really prove that the root-tip is sensitive to mere contact, since a certain amount of injury to the tissues was inflicted by the method employed; and this objection has not so far been fully met. Whatever may be the true explanation, it is a fact that roots find their way into worm-burrows, and otherwise follow in the earth lines of least resistance, in a way that is strongly suggestive of a power to discriminate between harder and softer regions of the soil.
Fig. 7.—A Seedling of Pea, with radicle extended horizontally in damp air, with a little square of card affixed to the lower side of its tip, causing it to bend upward in opposition to gravity. The deflection of the radicle after twenty-one hours is shown at A, and of the same radicle after forty-five hours at B. (From Darwin's Power of Movement in Plants.
An electric current passed through the tip induces curvature, and in some cases roots have been found to bend away from the light. Although it can hardly be supposed that sensitiveness to these stimuli is of any special use to the plants, such behavior, taken in connection with the highly useful modes of sensitiveness above described, surely indicates an almost animal-like irritability of the organ in question.
From what has been said of the curvature of young roots, it is obvious that, whenever the tip proper is stimulated, the effort must be transmitted to the part above, since it is only this upper portion which curves. A similar transmission of stimulus takes place in the leaf of the sensitive-plant, and both suggest an analogy with the propagation of an impulse along the nerves in animals. Nevertheless, in the absence of all proof that anything resembling nerves entered into the structure of plants, the analogy referred to was deemed rather fanciful, and certain mechanical explanations of the phenomena were offered as more in keeping with what was known. A few years ago, however, Gardiner's demonstration of the continuity of protoplasm in plants rendered the mechanical theories superfluous, by showing that the living matter of adjacent cells was connected by delicate protoplasmic threads which might fairly be considered the analogues of nerves. The essential similarity of many plant movements with those of animals is thus seen to be even closer than was at first supposed, and an added significance is given to the following words of Darwin, with which he closes his memorable work: "We believe that there is no structure in plants more wonderful, as far as its functions are concerned, than the tip of the radicle. If the tip be lightly pressed, or burnt or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side; and, what is more surprising, the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides. If, however, the radicle is pressed by a similar object a little above the tip, the pressed part does not transmit any influence to the more distant parts, but bends abruptly toward the object. If the tip perceives the air to be moister on one side than on the other, it likewise transmits an influence to the upper adjoining part, which bends toward the source of moisture. When the tip is excited by light, . . . the adjoining part bends from the light; but when excited by gravitation, the same part bends toward the center of gravity. In almost every case we can clearly perceive the final purpose or advantage of the several movements. Two, or perhaps more, of the exciting causes often act simultaneously on the tip, and the one conquers the other, no doubt in accordance with its importance for the life of the plant. The course pursued by the radicle in penetrating the ground must be determined by the tip; hence it has acquired such diverse kinds of sensitiveness. It is hardly an exaggeration to say that the tip of the radicle thus endowed, and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements."