Makers of British Botany; a collection of biographies by living botanists

by Unknown

Physiology.

The eighteenth century saw the birth of vegetable physiology, Hales and Knight being the two great pioneers in this country. The former flourished in the early part of the century, whilst Knight, although born in 1758, published his great work in 1806.

The chief physiological work of Hill is embodied in a pamphlet of 59 pages, entitled The Sleep of Plants and Causes of Motion in the Sensitive Plant explain'd, published in London in 1757, a year previous to the appearance of Du Hamel's Physique des Arbres. The paper is in the form of a letter to Linnaeus, and in it the author explains his position with regard to his earlier criticisms of the Linnaean system of classification.

The work is divided into sections, the first of which consists of a brief historical resumé, the opinions of Acosta, Alpinus, Ray and Linnaeus on this subject being alluded to. No mention, however, is made of the observations of Bonnet and of Mairan to the effect that the periodic movements of Mimosa pudica continued when the plant was kept in prolonged darkness.

In Section 2, after describing the structure of a leaf, Hill remarks that "Leaves are always surrounded by the air; and they are occasionally and variously influenced by heat, light, and moisture. They are naturally complicated, and they act on most occasions together. We are therefore to observe, first, what effects result from their mutual combinations in a state of nature: and having assigned in these cases the effect to the proper and particular cause, from this power of that agent, whichsoever it is, that acts thus in concert with the rest, we may deduce its operations singly."

This passage, although not particularly clear, indicates that Hill fully appreciated the fact that the reaction exhibited by a plant organ is a response to the resultant of a number of forces, and that each factor must be examined separately.

He then goes on to describe his observations on Abrus; the structure of the leaf, more especially the course of the vascular bundles, is first dealt with, and then an explanation of the action of light is given. Needless to say, in view of the state of physical science at this period, his explanation, although ingenious, is wide of the mark. He wrote that "Light is subtile, active, and penetrating: by the smallness of its constituent parts, it is capable of entering bodies; and by the violence of its motion, of producing great effects and changes in them. These are not permanent, because those rays which occasion them, are, in that very action, extinguished and lost.

"Bodies may act on light without contact; for the rays may become reflected when they come extreamly near: but light can act on bodies only by contact; and in that contact the rays are lost. The change produced in the position of the leaves of plants by light, is the result of a motion occasioned by its rays among their fibres: to excite this motion, the light must touch those fibres; and where light touches, it adheres, and becomes immediately extinguished.... The raising of the lobes in these leaves will be owing to the power of those rays which at any one instance fall upon them: these become extinguished; but others immediately succeed to them, so long as the air in which the plants stands, is enlightened."

Although it was not until 1822, when Dutrochet pointed out the true significance of the pulvini, Hill recognized that these structures were concerned with the movements of the leaflets, not only in the case of Abrus, but also in Mimosa. He remarked that "It is on the operation of light upon these interwoven clusters of fibres [which are placed at the bases of the main rib, and of the several foot-stalks of the lobes], that the motion of the leaves in gaining their different positions depends; and consequently, the motion itself is various according to the construction of these fibres.

"In the Abrus they are large, and of a lax composition; consequently the lobes are capable of a drooping, an horizontal, and an oblique upward position: in the Tamarind, and the broad-leaved Robinia, they are more compact, and hence all the motion of which those leaves are capable, is an expanding open and a closing sideways; which the direction and course of the fibres also favours: in the Parkinsonia they are smaller, and yet more compact; and the consequence of this is, that its lobes have no farther possible motion, than the expanding upwards."

Again, "The clusters of fibres are as a kind of joints on which their lobes are capable, under the influence of light, of a certain limited motion."

Further, with regard to Mimosa, he remarks that "To propagate the motion when the leaves are in a state to shew it, there requires a perfect and confirmed state of those clusters of fibres lodged at their base." Hill then describes the experiments upon which he based his conclusions; these shew that he was fully awake to the importance of keeping the conditions of an experiment, other than those of light, as near constant as possible, and that the position assumed by the leaves depends upon the intensity of the light.

His final experiment was to place the Abrus in a bookcase in such a position that the sun shone full upon it; when the leaves were fully expanded he closed the doors and found that in an hour "The lobes were all drop't, and it was in the same state that it would have shewn at midnight. On reopening the doors the elevated position of the leaves was assumed in twenty minutes."

Hill offers the same explanation of the movements of Mimosa as of those exhibited by Abrus, the reason for their greater conspicuousness in the former plant being due to the fact that in Mimosa "As there are no less than three sets of these clusters [of fibres which are placed at the bases of the foot-stalks], the effects of the same principle are naturally much greater than in the Abrus where there is only one."

Hill carefully observed the sequence of motion in the Mimosa, and points out that the effect of absolute darkness on the plant is greater than the rudest touch. He also found that the contact stimulus must be of a sufficient intensity, and that the degree of the subsequent motion depended upon the potency of the stimulus. He further observed that shaking the plant had the same effect as contact stimulation; also he remarks upon the fact that the movements of the Mimosa and of the Tamarind are less well-marked at a temperature lower than that in which the plants have been reared. Hill considered that "This is probably due to the juices stagnating in the clusters of fibres, and to the contraction of the bark by cold." His explanation of the response to the contact stimulus is of course quite wrong; it may, however, be quoted as an illustration of the view, current at that time, that such motion was due to the fibres which acted like those of muscle. "The vibration of the parts is that which keeps the leaves of the sensitive plant in their expanded and elevated state: this is owing to a delicate motion continued through every fibre of them. When we touch the leaf, we give it another motion more violent than the first: this overcomes the first: the vibration is stopped by the rude shock: and the leaves close, and their foot stalks fall, because that vibrating motion is destroyed, which kept them elevated and expanded.... That the power of motion in the sensitive plant depends upon the effect of light on the expanded surface of the leaves, is certain; for till they are expanded, they have no such power. The young leaves, even when grown to half an inch in length have no motion on the touch, tho' rough and sudden."

Hill fully appreciated the importance of comparative observations; he compared the movements, in response to light, of Abrus and Mimosa, which plants he placed side by side so that the conditions of the experiment might be the same for each. He found that "In these and in all others, the degree of elevation or expansion in the lobes, is exactly proportional to the quality of the light: and is solely dependent upon it."

Reference also may be made to Hill's views on reproduction[78]; he considered that the pollen grain contained the embryo which was set free by the bursting of the grain after it had been deposited upon the stigma. The stigmatic hairs or papillae were supposed to be the ends of tubes into which the embryos entered, made their way into the placenta, and thus arrived into the "shells of the seeds" (the ovules). It is unnecessary to point out the absurdities of these ideas, but it may be mentioned that Hill's interpretations of his observations were at fault rather than the observations themselves. Thus, judging from his figures, he saw the contents of the pollen grain, the appearance of which, under the conditions of observation, might easily suggest the idea of an embryo. Also he noticed that the pollen grains burst in a little while when placed in water, a phenomenon which was rediscovered 138 years later[79], and he therefore thought that a similar bursting, with a consequent setting free of the embryo, would take place on the wet stigma of the lily, for example.