TRANSACTIONS OF SECTION D. fifiS 



' Text-Boot,' edit. 4, 1874, under the same heading as the influence of light on 

 rectilinear growth. 



Shortly afterwards, in 1876, a pupil of Sachs, Miiller-Thurgau, published ' a 

 research carried out in the WUrzhurii: Laboratory, which is of some importance. 

 ]n the introductory remarks he wrote : — ' It has been hitherto supposed that helio- 

 tropic curvatures depend on a difference in intensity of illumination on the two 

 sides. Sachs came to a different opinion in his work on geotropism : he found 

 himself compelled to believe that in heliotropic just as in geotropic curvatures it is 

 not a question of different intensities on opposite sides, but rather that heliotropic 

 effect depends on the direction of the light.' - 



Miiller's research gave weight to this union of geo- and heliotropic effects by 

 showing a number of resemblances in the manner and form of the two curvatures. 

 Again, when it was found ^ that apheliotropie organs are influenced by light and 

 darkness in precisely the same manner as positively heliotropic ones, it became 

 clear that the mechanical explanation of De Candolle was untenable for negatively 

 heliotropic organs. It might still no doubt be upheld for positively heliotropic 

 organs, but as a matter of fact it was not so upheld. There was a tendency 

 to unify our view of growth-curvatures, and the union of the two forms of 

 heliotropism gave strength to the movement. Nor was this all ; when it became 

 clear that light did not produce heliotropic curvatures by direct mechanicaleffect 

 it was natural to remember that gravitation has none either ; we cannot point to 

 any reason (except the crudest ones) why the lower side of a horizontal stem, or 

 the upper side of a horizontal root, should grow the faster for the direct effects of 

 gravitation. That being so, light and gravitation could be classed together as 

 external agencies acting, not directly, but in some unknown indirect manner. I do 

 not imply that such a result followed immediately, but that the line of research 

 above alluded to helped in some degree to lead the way to a belief in growth- 

 curvatures as phenomena of irritability. 



When my father was writing our book, ' The Power of Movement in Plants ' 

 (1880), in which he adopted to the fullest extent a belief that growth curvatures 

 are phenomena of irritability, the only modern statement of such a view which he 

 could find was in a passage by Sachs,* where he writes that ' The living material of 

 plants is internally difl'erentiated in such a way that diflerent parts are supplied 

 with specific energies resembling those of the sensory-nerves (Sinnesnerven) of 

 animals. Anisotropy in plants fulfils the same purpose as do sense-perceptions in 

 animals.' 



The idea of irritability as applied to growth curvatures is expressed with suffi- 

 cient clearness in the ' Power of ^Movement.' Thus for the case of geotropism we 

 wrote (p. 521) : ' Different parts or organs on the same plant, and the same part in 

 different species, are thus excited to act in a widely different manner. AVe can see 

 no reason why the attraction of gravity should directly modify the state of 

 targescence and subsequent growth of one part on the upper side and of another 

 part on the lower side. We are therefore led to infer that both geotropic, apogeo- 

 tropic, and diageotropic movements, the purpose of which we can generally 

 understand, have been acquired for tlie advantage of the plant by the modification 

 of the ever present movement of circumnutation. This, however, implies that 

 gravitation produces some effect on the young tissues sufficient to serve as a 

 guide to the plant.' A similar view is given for heliotropism. It should be noted 

 that the essence of the view, namely, that light and gravitation act as guides or 

 landmarks by which the plant can "direct itself, can be held without a belief ia 

 circumnutation. 



In PfeS'er's admirable ' Pflanzenphysiologie,' 1881, the concrption of stunulus 



> Flora, 1876. 



- In his Vorlesungen, p. 854, Sachs states that he wrote Jliiller-Thurgau's intro- 

 duction. 



» Schmitz, Linntra, 184.3; Mullcr-Thurgau (Flora, 1876); F. Darwin, Sachs' 

 Arheiten, 1880. The two latter researches were carried out under the direction of 

 Sachs in his laboratorv. 



• Sachs (Arheiten, ii. 1870, p. 282). 



