4o8 



NATURE 



[August 27, 1891 



(i) How does the plant recognize 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 mechan- 

 ism of movement. Sachs has well pointed out that these two 

 very dilTerent questions have been confused together {Arbeiten, 

 ii. p. 282, 1879). 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 con- 

 veniently 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 [Berichte d. k. Sachs. Ges. d. Wiss.), 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 geo- 

 tropism formed by Hofmeister and accepted with some modi- 

 fication by Sachs. When an upright, vigorously-growing, 

 turgescent shoot is struck at its base the upper end is made to 

 curve violently towards the 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 

 Hofmei^ter 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 tis>ue 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 bein,;; 

 passively extended by the active tissue. Hofmeister showed 

 that when the shoot is violently bent the elasticity of the pa-sive 

 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 i860, Hofmester {Btrichte d. k. Sachs. 

 Ges. d. Wiss.) applied these principles to the explanation 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 i860. 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 ihe 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. 



His view s resembled more modern theories in this, too : he 

 clearly recognized that they were, mutatis mutandis, applicable 

 to acellular- organs. The manner in which Hofmeister com- 

 pared 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. 



' Kn ght had previously suggested an explanation (Philosophical 

 Tran-iaction-!, 1806 1, which is so far similar, that tlie sinking downwards by 

 gravitation of the juices of the plant is supposed to be the primary cause of 

 apogeotr jpism. Knight's explanation of positive geotropism is practically 

 the same as Hofmeister's. 



2 Sachs's term acellular is, in the present connection, equivalent to 

 untcellnlar. 



NO. 1 139, VOL. 44] 



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 localized 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 givep 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 apogeo- 

 tropic 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 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. Treviranu , in his " Physiologic " 

 (1838), speaks of geotropism as a lYicb, 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 some- 

 thing 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 generalization — namely, into separating 

 what are now universally considered parts of a single pheno- 

 menon—viz. negative and positive geotropism. He gave 

 totally difTerent explanations of the bending down of a root and 

 the bending up 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 in- 

 teresting. 



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 releairc of stored-up energy occurs, we apple 

 the term irritability to the phenomenon, and we call the influency 

 which produced the change a stimulus. At this time (1865) 

 there was, as far as I can discover, no idea that growth-curva- 

 tures 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 

 I'attraction n'est que la cause occasionelle du mouvement de- 

 scendant des racines et de I'ascension des tiges ; elle n'en est 

 point la cause immediate; elle agit dans cette circ instance 

 comme agent nervimoteur. Nous verrons plus bas de nouvelles 

 preuves de la generalite de ce fait important en physiologie. 

 savoir que les mouvements visibles des vegetaux s nt tous des 

 mouvements spontanes, executes a I'occasion de I'influence d'un 

 agent exterieur et non des mouvements im primes 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 



' " Recherches anat. sur la Structure intime, &c." (1824), p. 107. Dutro- 

 chet, however, was not consistent in this matter, and later on gave explana- 

 tions as mechanical as Hofmeister's. 



