2

In his latest paper, however, Mr. Waters,' after reviewing Mr. Ulrich's work on the Paleozoic Bryozoa of America, remarks (p. 53): 'I would urge the importance of a thorough comparison of Palæozoic with Cretaceous genera, for the number of known Cretaceous genera is very large, and with these and the present fauna comparison can be made, thus giving the best stepping-stone between the rich carboniferous fauna and the recent.' Such a comparison would be of great value to the paleontologist, but the comparison must be made with actual examples of the fauna from the several formations, and not with figures or mere descriptions of the same. During the last twenty years some thousands of examples from all formations have passed through my hands, and possibly some light could be thrown upon the question indicated by Mr. Waters, but ample space and special illustrations are necessary to do the work well.

1 Ann. Mag. Nat. Hist. s. t. vol. viii. p. 53, July 1891.

2 Geology and Palæontology, vol. viii. Geol. Survey, Illinois, 1890.

SECTION D.-BIOLOGY.

PRESIDENT OF THE SECTION-FRANCIS DARWIN, M.A., M.B., F.R.S.

THURSDAY, AUGUST 20.

The PRESIDENT delivered the following Address :-

On Growth-curvatures in Plants.

A SEEDLING plant, such as a young sunflower when growing in a state of nature, grows straight up towards the open sky, while its main root grows straight down towards the centre of the earth. When it is artificially displaced, for instance, by laying the flower-pot on its side, both root and stem execute certain curvatures by which they reach the vertical once more. Curvatures such as these, whether executed in relation to light, gravitation, or other influences, may be grouped together as growth-curvatures, and it is with the history of our knowledge on this subject that I shall be occupied to-day. I shall principally deal with geotropic curvatures, or those executed in relation to gravitation, but the phenomena in question form a natural group, and it will be necessary to refer to heliotropism and, indeed, to other growth-curvatures. The history of the subject divides into two branches, which it will be convenient to study separately.

When a displaced apogeotropic organ curves so as to become once more vertical, two distinct questions arise, which may be briefly expressed thus:

1. How does the plant recognise the vertical line; how does it know where the centre of the earth is ?

2. In what way are the curvatures which bring it into the vertical line executed ?

The first is a question of irritability, the second of the mechanism of movement. Sachs has well pointed out that these two very different questions have been confused together. They should be kept as distinct as the kindred questions how. by what nervous apparatus, does an animal perceive changes in the external world; and how, by what muscular machinery, does it move in relation to such changes?

The history of our modern knowledge of geotropism may conveniently begin with Hofmeister's researches, because in an account of his work some of the points which re-occur in recent controversy are touched, and also because in studying his work the necessity of dividing the subject into the two above-named headings, Irritability and Mechanism will be more clearly perceived.

In 1859 Hofmeister published his researches on the effect of disturbance, such as shaking or striking a turgescent shoot. This appears at first sight sufficiently remote from the study of geotropism, but the facts published in this work were the basis of the theory of geotropism formed by Hofmeister and accepted with some modification by Sachs. When an upright, vigorously-growing, turgescent shoot is struck at its base the upper end is made to curve violently towards the Arbeiten, ii. p. 282 (1879).

2 Hofmeister, Berichte d. k. Sächs. Ges. d Wiss., 1859.

side from which the blow came.

When the shoot comes to rest it is found to be no longer straight, but to have acquired a permanent bend towards the side on which it was struck. In explaining this phenomenon Hofmeister described those conditions of growth which give rise to what is known as the tension of tissues: these facts are still an important part of botanical study, though they hold quite a different position from that assigned to them by Hofmeister. The classification into active or erectile tissue and passively extended tissue was then first made. The pith, which is compressed, and strives to become longer, is the active or erectile part, the cortical and vascular constituents being passively extended by the active tissue. Hofmeister showed that when the shoot is violently bent the elasticity of the passive tissues on the convex side is injured by overstretching. The system must assume a new position of equilibrium; the passive tissues are now no longer equally resisting on the two sides, and the shoot must necessarily assume a curvature towards that side on which passive tissues are most resisting.

In a second paper, in 1860, Hofmeister1 applied these principles to the explana. tion of geotropism. It is true that in his second paper he does not refer to the former one, but I think that it can hardly be doubted that the knowledge which supplied the material for his paper of 1859 suggested the theory set forth in 1860. He had shown that in the system of tensions existing in a turgescent shoot lay the power of producing artificial curvatures, and he applied the same principle to the natural curvatures. When an apogeotropic organ is placed in a horizontal position Hofmeister supposed that the resisting tissues on the lower side became less resisting, so that they yielded more readily than those on the upper side to the longitudinal pressure of the turgescent pith. The system in such a case comes to rest in a new position, the shoot curving upwards; the passive tissues on the upper and lower sides once more resist the expansion of the pith in equal degrees. In this way Hofmeister hit on an explanation which, as far as mechanism is concerned, is in rough outline practically the same as certain modern theories, which will be discussed in the sequel.

3

His views resembled more modern theories in this, too: he clearly recognised that they were, mutatis mutandis, applicable to acellular organs. The manner in which Hofmeister compared the mechanics of multicellular and acellular parts was curious; nowadays we compare the turgescent pith of a growing shoot with the hydrostatic pressure inside the acellular organ. Just as the pressure inside a single cell stretches the cell-walls, so in a growing shoot the turgescent pith stretches the cortex. As pith is to cortex, so is cell-pressure to cell-membrane. But Hofmeister would not have accepted any such comparison. In the case of acellular organs he localised both the erectile and passive tissues in the membrane. The cuticle was said to be passively extended by the active growth of the inner layers of the cell-wall.

It is remarkable that the obvious source of power which the pressure of the cell-sap against the cell-walls supplies should have been so much neglected. This may perhaps be accounted for as a revulsion against the excessive prominence given to osmosis in the works of Dutrochet.

The great fault of Hofmeister's views was the purely mechanical manner in which he believed changes in extensibility in the passive tissues to be brought about. When an apogeotropic shoot is placed horizontal there would be a tendency, according to Hofmeister, for the resisting passive tissues along the lower side of the shoot to become waterlogged owing to the fluid in the shoot gravitating towards that side. They would thus be rendered more extensible, and the shoot would bend up, since its lower parts would yield to the erectile tissues in the centre. Such a conception excludes the idea of gravitation acting as a stimulus, and tends to keep geotropism out of the category in which it now takes its place

'Hofmeister, Berichte d. k. Sächs. Ges. d. Wiss. 1860.

Knight had previously suggested an explanation (Philosophical Transactions, 1806), which is so far similar, that the sinking downwards by gravitation of the juices of the plant is supposed to be the primary cause of apogeotropism. Knight's explanation of positive geotropism is practically the same as Hofmeister's.

Sachs' term acellular is, in the present connection, equivalent to unicellular.

along with such obvious cases of response to stimulation as the movements of Mimosa. In this respect it was a retrogression from the views of some earlier writers. Dutrochet's clear statement (1824) as to growth-curvatures being an affair of stimulus and response will be quoted lower down. Treviranus in his Physiologie' (1838) speaks of geotropism as a Trieb, or impulse, and adds that though there is no question of desire or sensation as in the impulses of animals, yet geotropism must be thought of as something higher than a merely mechanical or chemical action,

In taking such a view Hofmeister naturally neglected the biological side of the study of geotropism. Now we think of gravitation as a stimulus, which the plant translates according to its needs. The plant, so to speak, knows where the centre of the earth is, and either grows away from it, or towards it, according as either direction suits its mode of existence.

We have seen how Hofmeister's view enabled him to apply a common explanation to acellular and multicellular organisms. But it led him into an error which more than counterbalances the credit due to such a generalisation, namely, into separating what are now universally considered parts of a single phenomenon-viz., negative and positive geotropism. He gave totally different explanations of the bending down of a root and the bending up of a stem. It is well known that he supposed a root to be plastic, and to bend over by its own weight, like a tallow candle on a hot day or a piece of heated sealing-wax.

The development of a unified view of heliotropism, geotropism, and other similar curvatures is a part of my subject, and for that reason the curious want of unity in Hofmeister's views is interesting.

In 1865 Sachs published his Experimental-Physiologie.' He here accepts Hofmeister's views with certain modifications.

Irritability.

When by a touch on a trigger the explosion of a pistol is caused we do not say that the pistol is irritable, but when in an organism a similar release of stored-up energy occurs we apply the term irritability to the phenomenon, and we call the influence which produced the change a stimulus. At this time (1865) there was, as far as I can discover, no idea that growth-curvatures were produced by external influences acting as stimuli. Gravitation and light were supposed to act directly, and not as releasing forces. This is all the more remarkable, because Dutrochet had expressed with great clearness the conception which we now hold. He wrote:-'La cause inconnue de l'attraction n'est que la cause occasionelle du mouvement descendant des racines et de l'ascension des tiges; elle n'en est point la cause immédiate; elle agit dans cette circonstance comme agent nervimoteur. Nous verrons plus bas de nouvelles preuves de la généralité de ce fait important en physiologie, savoir que les mouvements visibles des végétaux sont tous des mouvements spontanés, exécutés à l'occasion de l'influence d'un agent extérieur et non des mouvements imprimés par cet agent.' Nothing could be more to the purpose than this, and it is one of the most curious points in the history of the subject that the botanical mind should have taken more than fifty years to assimilate Dutrochet's view.

In 1868 Albert Bernhard Frank published his valuable 'Beiträge zur Pflanzenphysiologie,' which was of importance in more than one way. In this work the term 'geotropism' was first suggested in imitation of the existing expression 'heliotropism.' This uniformity of nomenclature had an advantage beyond mere convenience, for it served to emphasise the view that the curvatures were allied in character. His criticisms of Hofmeister and Sachs were directed against the following views:-(i.) That roots and other positively geotropic organs_bend owing to plasticity. By repeating and varying certain older experiments, Frank helped materially to establish the now universally accepted view that positive

1 Recherches anat. sur la Structure intime, &c. 1824, p. 107. Dutrochet, however, was not consistent in this matter, and later on gave explanations as mechanical as Hofmeister's.

geotropism is an active, not a passive, curvature, and that it depends, like apogeotropism, on unequal distribution of longitudinal growth. Here, again, he introduced unity, bringing what had been considered different phenomena under a common heading. By studying the distribution of growth and of tension in a variety of curvatures he helped still more to unite them under a common point of view.

(ii.) He showed that Hofmeister's classification of organs into those (1) which have and (2) which have not tension, was valueless in connection with growthcurvatures; that is to say that apogeotropism is not necessarily connected with the form of longitudinal tension found in growing shoots, and that the distinct kind of tension existing in roots has no connection with their positive geotropism. His work thus served to bring the subject into a more purely physiological condition, not only by his downright opposition to a mechanical theory backed by the great name of Hofmeister, but also by giving importance to physiological individuality.

In 1870 Frank published a more important work, Die natürliche wagerechte Richtung der Pflanzentheilen.' This paper not only tended to unite geotropism and heliotropism by proving the phenomena described to be common to both categories, but it more especially widened the field of view by showing that horizontal growth must be considered as kindred to vertical growth, and thus introduced a new conception of the reaction of plants to light and gravitation which has been most fruitful.

Frank showed that certain parts of plants, for instance the runners of the strawberries, even when kept in the dark, grow horizontally, and when displaced from the horizontal returned to it. Here, said Frank, is a new type of geotropism, neither positive nor negative, but transverse. Ten years later Elfving, working in Sachs' laboratory, got similar results with rhizomes of Scirpus, &c. These experiments are more conclusive than Frank's in one way, because the strawberry runners when darkened were in abnormal conditions, whereas the rhizomes used by Elfving were normally freed from light-effects. When a rhizome which has been placed so as to point obliquely upwards moves down towards the horizontal position it is, according to the old nomenclature, positively geotropic, and, vice versa, when it reaches the horizontal from below it is negatively geotropic. But it cannot be both positively and negatively geotropic. We are bound to assume that it is so organised that it can only assume a position of rest, and continue to grow in a straight line when it is horizontal, just as an ordinary geotropic organ cannot devote itself to rectilinear growth unless it is vertical. In this way Frank's conception of transverse geotropism paved the way for the theory that there are a variety of different organisations (or, as we may now say, irritabilities) in growing plants; and that, whether a plant grows vertically upwards or downwards or horizontally, depends on the individual and highly sensitive constitution of the plant in question. It is, of course, true that those who seek for mechanical explanations of growth curvatures might be able to find such a one for transverse geotropism. But when Frank's conception has once been seized such views are less and less acceptable; and, judging from my own experience, I cannot doubt that Frank's work deserved to have a powerful effect in preparing the minds of physiologists for a just view of irritability.

The belief in transverse geotropism received interesting support from Vöchting's2 work on the movement of certain flowers which retain a horizontal position under the influence of gravitation.

Frank's views, it may be added, were accepted by my father and myself in our 'Power of Movement,' in which the term diageotropism was proposed, and has been generally accepted, for transverse geotropism. Nevertheless, though Frank was undoubtedly right, his views were strongly opposed at the time. He held similar views on the effect of light, believing that the power possessed by leaves of placing themselves at right angles to the direction of incident light must be considered as a new type of heliotropic movement, transverse or diaheliotropism. Frank's views were criticised and opposed by De Vries, who, by means of experiElfving, Sachs' Arbeiten, 1880. 2 Die Bewegung der Blüthen und Früchte, 1882. 3 De Vries, Sachs' Arbeiten, 1872.