PLANT MOVEMENT
The broad bean or fava bean, a culinary staple since antiquity, is, botanically speaking, a species of vetch, belonging to a group of nearly 150, mainly annual, climbing legumes in the genus Vicia. The family name Fabaceae is a vestige of botanical history—during a brief time when these beans were placed in genus Faba (Latin for “bean”) they were selected as the taxonomic representative of legumes—making Faba faba, the “bean of beans,” the representative species—and so the genus was built into the family name. The genus name Faba has long since been defunct, replaced with Vicia, but it lives on in the higher classification. That’s the sort of relict, so to speak, that Darwin found instructive as an analog of evolutionary history—like the way oddities of spelling or silent letters can reveal the origin and evolution of certain words. But this nomenclatural example didn’t exist in Darwin’s day, when legumes were variously known by the more descriptive names “Leguminosae” or “Papilionaceae.” His interest in them had more to do with pollination, movement, and physiology.
In the course of his investigations into the fertility of cultivated beans, Darwin found that flowers of fava beans have to be cross-
pollinated by insects to ensure proper seed set. He demonstrated this in straightforward experiments, such as the one in which he netted one group of fava beans in his garden to exclude bees and left others open, harvesting 135 beans from seventeen “open pollinated” plants, compared to a measly 40 beans from a like number of covered ones. But the study of fava bean pollination was minor compared to his years-long focus on the many forms of movement in this species, including circular-motion circumnutation of leaves, where he observed that “both the whole leaf and the terminal leaflets undergo a well-marked daily periodical movement, rising in the evening and falling during the latter part of the night.” Even more elaborate were studies of seedlings, testing circumnutation of the epicotyl (the stem region above the cotyledon of a seedling), radicle (the embryonic root), and hypocotyl (the region between the cotyledon and radicle). Readily available and easy to germinate, fava beans were ideal for such research.
Darwin and his son Francis carefully traced the movement of fava bean radicles as they grew, and, by adhering tiny card rectangles in different locations near the apex, made the surprising discovery that the radicles are touch-sensitive—they tend to curve away from the side touched, just the opposite of the reaction observed with tendrils. These and other experiments, such as the reaction of radicle tips to light and gravity, led the father-son team to declare that “there is no structure in plants more wonderful, as far as its functions are concerned, than the tip of the radicle.” But how can cells at the tip of this structure influence cell growth further up the elongating root stem? They marveled that the tip acts somewhat like a brain. In the very last sentence of Movement they developed this intriguing analogy: “It is hardly an exaggeration to say that the tip of the radicle thus endowed [with sensitivity] 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.”148 Later known as Darwin’s “root-brain hypothesis” and largely discounted in the twentieth century, this idea has re-emerged in recent years in the form of a thought-provoking “phytoneurobiological” model for understanding plant growth and physiology.149 When next you sit down to a hearty dish of fava beans, raise a fork to this humble bean that has not only played a leading role in the development of the field of plant physiology but continues to instruct as a staple, so to speak, of high-school and college botany classes today.
In order to see how the radicles of seedlings would pass over stones, roots, and other obstacles, which they must incessantly encounter in the soil, germinating beans (Vicia faba) were so placed that the tips of the radicles came into contact, almost rectangularly or at a high angle, with underlying plates of glass. In other cases, the beans were turned about whilst their radicles were growing, so that they descended nearly vertically on their own smooth, almost flat, broad upper surfaces. The delicate root-cap, when it first touched any directly opposing surface, was a little flattened transversely; the flattening soon became oblique, and in a few hours quite disappeared, the apex now pointing at right angles, or at nearly right angles, to its former course. The radicle then seemed to glide in its new direction over the surface which had opposed it, pressing on it with very little force. How far such abrupt changes in its former course are aided by the circumnutation of the tip must be left doubtful. Thin slips of wood were cemented on more or less steeply inclined glass-plates, at right angles to the radicles which were gliding down them. Straight lines had been painted along the growing terminal part of some of these radicles, before they met the opposing slip of wood; and the lines became sensibly curved in 2 h. after the apex had come into contact with the slips. In one case of a radicle, which was growing rather slowly, the root-cap, after encountering a rough slip of wood at right angles, was at first slightly flattened transversely: after an interval of 2 h. 30 m. the flattening became oblique; and after an additional 3 hours the flattening had wholly disappeared, and the apex now pointed at right angles to its former course. It then continued to grow in its new direction alongside the slip of wood, until it came to the end of it, round which it bent rectangularly. Soon afterwards when coming to the edge of the plate of glass, it was again bent at a large angle, and descended perpendicularly into the damp sand. …
… An object which yields with the greatest ease will deflect a radicle: thus, as we have seen, when the apex of the radicle of the bean encountered the polished surface of extremely thin tin-foil laid on soft sand, no impression was left on it, yet the radicle became deflected at right angles. A second explanation occurred to us, namely, that even the gentlest pressure might check the growth of the apex, and in this case growth could continue only on one side, and thus the radicle would assume a rectangular form; but this view leaves wholly unexplained the curvature of the upper part, extending for a length of 8–10 mm.
We were therefore led to suspect that the apex was sensitive to contact, and that an effect was transmitted from it to the upper part of the radicle, which was thus excited to bend away from the touching object. As a little loop of fine thread hung on a tendril or on the petiole of a leaf-climbing plant, causes it to bend, we thought that any small hard object affixed to the tip of a radicle, freely suspended and growing in damp air, might cause it to bend, if it were sensitive, and yet would not offer any mechanical resistance to its growth. Full details will be given of the experiments which were tried, as the result proved remarkable. The fact of the apex of a radicle being sensitive to contact has never been observed, though, as we shall hereafter see, Sachs discovered that the radicle a little above the apex is sensitive and bends like a tendril towards the touching object. But when one side of the apex is pressed by any object, the growing part bends away from the object; and this seems a beautiful adaptation for avoiding obstacles in the soil, and, as we shall see, for following the lines of least resistance. Many organs, when touched, bend in one fixed direction, such as the stamens of Berberis, the lobes of Dionaea, etc.; and many organs, such as tendrils, whether modified leaves or flower-peduncles, and some few stems, bend towards a touching object; but no case, we believe, is known of an organ bending away from a touching object.
Sensitiveness of the Apex of the Radicle of Vicia faba.—Common beans, after being soaked in water for 24 h., were pinned with the hilum downwards (in the manner followed by Sachs), inside the cork lids of glass-vessels, which were half filled with water; the sides and the cork were well moistened, and light was excluded. As soon as the beans had protruded radicles, some to a length of less than a tenth of an inch, and others to a length of several tenths, little squares or oblongs of card were affixed to the short sloping sides of their conical tips. The squares therefore adhered obliquely with reference to the longitudinal axis of the radicle; and this is a very necessary precaution, for if the bits of card accidentally became displaced, or were drawn by the viscid matter employed so as to adhere parallel to the side of the radicle, although only a little way above the conical apex, the radicle did not bend in the peculiar manner which we are here considering. Squares of about the 1\20th of an inch (i.e. about 1½ mm.), or oblong bits of nearly the same size, were found to be the most convenient and effective. We employed at first ordinary thin card, such as visiting cards, or bits of very thin glass, and various other objects; but afterwards sand-paper was chiefly employed, for it was almost as stiff as thin card, and the roughened surface favoured its adhesion. At first we generally used very thick gum-water; and this of course, under the circumstances, never dried in the least; on the contrary, it sometimes seemed to absorb vapour, so that the bits of card became separated by a layer of fluid from the tip. When there was no such absorption and the card was not displaced, it acted well and caused the radicle to bend to the opposite side. I should state that thick gum-water by itself induces no action. In most cases the bits of card were touched with an extremely small quantity of a solution of shellac in spirits of wine, which had been left to evaporate until it was thick; it then set hard in a few seconds and fixed the bits of card well. When small drops of the shellac were placed on the tips without any card, they set into hard little beads, and these acted like any other hard object, causing the radicles to bend to the opposite side. …
… As the chief curvature of the radicle is at a little distance from the apex, and as the extreme terminal and basal portions are nearly straight, it is possible to estimate in a rough manner the amount of curvature by an angle; and when it is said that the radicle became deflected at any angle from the perpendicular, this implies that the apex was turned upwards by so many degrees from the downward direction which it would naturally have followed, and to the side opposite to that to which the card was affixed. That the reader may have a clear idea of the kind of movement excited by the bits of attached card, we append here accurate sketches of three germinating beans thus treated and selected out of several specimens to show the gradations in the degrees of curvature.
Vicia faba: A, radicle beginning to bend from the attached little square of card; B, bent at a rectangle; C, bent into a circle or loop, with the tip beginning to bend downwards through the action of geotropism.
Vinca minor. Watercolor by artist at French School, An Album of Flowers.