a dozen or so taped sessions of the Beatles revising “Strawberry

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Another subject on which a good deal of labour was expended is the sleep of leaves, or as Darwin called it their NYCTITROPIC movement. He showed for the first time how widely spread this phenomenon is, and attempted to give an explanation of the use to the plant of the power of sleeping. His theory was that by becoming more or less vertical at night the leaves escape the chilling effect of radiation. Our method of testing this view was to fix some of the leaves of a sleeping plant so that they remained horizontal at night and therefore fully exposed to radiation, while their fellows were partly protected by assuming the nocturnal position. The experiments showed clearly that the horizontal leaves were more injured than the sleeping, i.e. more or less vertical, ones. It may be objected that the danger from cold is very slight in warm countries where sleeping plants abound. But it is quite possible that a lowering of the temperature which produces no visible injury may nevertheless be hurtful by checking the nutritive processes (e.g. translocation of carbohydrates), which go on at night. Stahl ("Bot. Zeitung", 1897, page 81.) however has ingeniously suggested that the exposure of the leaves to radiation is not DIRECTLY hurtful because it lowers the temperature of the leaf, but INDIRECTLY because it leads to the deposition of dew on the leaf-surface. He gives reasons for believing that dew-covered leaves are unable to transpire efficiently, and that the absorption of mineral food-material is correspondingly checked. Stahl's theory is in no way destructive of Darwin's, and it is possible that nyctitropic leaves are adapted to avoid the indirect as well as the direct results of cooling by radiation.

a dozen or so taped sessions of the Beatles revising “Strawberry

In what has been said I have attempted to give an idea of some of the discoveries brought before the world in the "Power of Movement" (In 1881 Professor Wiesner published his "Das Bewegungsvermogen der Pflanzen", a book devoted to the criticism of "The Power of Movement in Plants". A letter to Wiesner, published in "Life and Letters", III. page 336, shows Darwin's warm appreciation of his critic's work, and of the spirit in which it is written.) and of the subsequent history of the problems. We must now pass on to a consideration of the central thesis of the book,--the relation of circumnutation to the adaptive curvatures of plants.

a dozen or so taped sessions of the Beatles revising “Strawberry

Darwin's view is plainly stated on pages 3-4 of the "Power of Movement". Speaking of circumnutation he says, "In this universally present movement we have the basis or groundwork for the acquirement, according to the requirements of the plant, of the most diversified movements." He then points out that curvatures such as those towards the light or towards the centre of the earth can be shown to be exaggerations of circumnutation in the given directions. He finally points out that the difficulty of conceiving how the capacities of bending in definite directions were acquired is diminished by his conception. "We know that there is always movement in progress, and its amplitude, or direction, or both, have only to be modified for the good of the plant in relation with internal or external stimuli."

a dozen or so taped sessions of the Beatles revising “Strawberry

It may at once be allowed that the view here given has not been accepted by physiologists. The bare fact that circumnutation is a general property of plants (other than climbing species) is not generally rejected. But the botanical world is no nearer to believing in the theory of reaction built on it.

If we compare the movements of plants with those of the lower animals we find a certain resemblance between the two. According to Jennings (H.S. Jennings, "The Behavior of the Lower Animals". Columbia U. Press, N.Y. 1906.) a Paramoecium constantly tends to swerve towards the aboral side of its body owing to certain peculiarities in the set and power of its cilia. But the tendency to swim in a circle, thus produced, is neutralised by the rotation of the creature about its longitudinal axis. Thus the direction of the swerves IN RELATION TO THE PATH of the organism is always changing, with the result that the creature moves in what approximates to a straight line, being however actually a spiral about the general line of progress. This method of motion is strikingly like the circumnutation of a plant, the apex of which also describes a spiral about the general line of growth. A rooted plant obviously cannot rotate on its axis, but the regular series of curvatures of which its growth consists correspond to the aberrations of Paramoecium distributed regularly about its course by means of rotation. (In my address to the Biological Section of the British Association at Cardiff (1891) I have attempted to show the connection between circumnutation and RECTIPETALITY, i.e. the innate capacity of growing in a straight line.) Just as a plant changes its direction of growth by an exaggeration of one of the curvature-elements of which circumnutation consists, so does a Paramoecium change its course by the accentuation of one of the deviations of which its path is built. Jennings has shown that the infusoria, etc., react to stimuli by what is known as the "method of trial." If an organism swims into a region where the temperature is too high or where an injurious substance is present, it changes its course. It then moves forward again, and if it is fortunate enough to escape the influence, it continues to swim in the given direction. If however its change of direction leads it further into the heated or poisonous region it repeats the movement until it emerges from its difficulties. Jennings finds in the movements of the lower organisms an analogue with what is known as pain in conscious organisms. There is certainly this much resemblance that a number of quite different sub-injurious agencies produce in the lower organisms a form of reaction by the help of which they, in a partly fortuitous way, escape from the threatening element in their environment. The higher animals are stimulated in a parallel manner to vague and originally purposeless movements, one of which removes the discomfort under which they suffer, and the organism finally learns to perform the appropriate movement without going through the tentative series of actions.

I am tempted to recognise in circumnutation a similar groundwork of tentative movements out of which the adaptive ones were originally selected by a process rudely representative of learning by experience.

It is, however, simpler to confine ourselves to the assumption that those plants have survived which have acquired through unknown causes the power of reacting in appropriate ways to the external stimuli of light, gravity, etc. It is quite possible to conceive this occurring in plants which have no power of circumnutating--and, as already pointed out, physiologists do as a fact neglect circumnutation as a factor in the evolution of movements. Whatever may be the fate of Darwin's theory of circumnutation there is no doubt that the research he carried out in support of, and by the light of, this hypothesis has had a powerful influence in guiding the modern theories of the behaviour of plants. Pfeffer ("The Physiology of Plants", Eng. Tr. III. page 11.), who more than any one man has impressed on the world a rational view of the reactions of plants, has acknowledged in generous words the great value of Darwin's work in the same direction. The older view was that, for instance, curvature towards the light is the direct mechanical result of the difference of illumination on the lighted and shaded surfaces of the plant. This has been proved to be an incorrect explanation of the fact, and Darwin by his work on the transmission of stimuli has greatly contributed to the current belief that stimuli act indirectly. Thus we now believe that in a root and a stem the mechanism for the perception of gravitation is identical, but the resulting movements are different because the motor-irritabilities are dissimilar in the two cases. We must come back, in fact, to Darwin's comparison of plants to animals. In both there is perceptive machinery by which they are made delicately alive to their environment, in both the existing survivors are those whose internal constitution has enabled them to respond in a beneficial way to the disturbance originating in their sense-organs.

By K. GOEBEL, Ph.D. Professor of Botany in the University of Munich.

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