TRANSACTIONS OF SECTION D. 661 



side from wliicli the blow came. When tlie 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 strucli. In explaining this phenomenon Ilofmeister described those 

 conditions of growtli 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 difterent position from that assigned to them by Hofmeister. The classifi- 

 cation 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 

 ov erectile part, the cortical and vaschlar constituents being pa.ssively extended 

 by the active tissue. Ilofmeister 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 18G0, Hofmeister ' 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 resist- 

 ing, so that they yielded more readily than those on the upper side to the longitu- 

 dinal 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 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 Hof- 

 meister would not have accepted any such comparison. In the case of acellular 

 organs be 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 tiuid 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, Bericltfc d. li. Siichs. Ges. d. Wiss. 1860. 



- Knight had previously suggested an explanation (PliilosopJdcal Transactions, 

 1 806), 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. 



