46 PBOCEKDINGS OF THE 



Tiic explanations hitherto given of the power possessed by 

 leaves of placing themselves at right angles to the direction of 

 incident light are: — (1) That of Frank; and (2) the opposite 

 theory of 8achs and De Vries. Frank's idea was that the leaves 

 are transversely heliotropic (or, as Mr. Darwin called it, diahelio- 

 troplc); that is, he supposed theai to be endowed with a specific 

 power of growth which is not as yet capable of further ana- 

 lysis, and which must at present be accepted just as geotropism, 

 for instance, is accepted as a term expressing the inexplicable 

 power which plants have of growing vertically upwards. Sachs 

 and De Vries, on tlie other hand, believe that the phenomena 

 can be explained as being due to the interaction of the ordinary 

 growth-tendencies exhibited by leaves. Of these only two 

 need here be mentioned, viz. apheliotropism and apogeotropism. 

 If we imagine a plant growing out of doors and illuminated from 

 the zenitii, then if its leaves are aplicliotropic, that is, if they 

 tend to curve away from the light, they will of course bend 

 downwards and point towards the ground ; but if they are at the 

 same time apogeotropic, they will tend to curve upwards away 

 from the ground. Now Sachs and De Vries suppose that the 

 horizontal position of leaves which are exposed to zenith illumi- 

 nation is due to a balance struck between some such opposing 

 growth-tendencies as those above sketched. Francis Darwin 

 has shown, however, that at least for certain plants, e. g. Ranun- 

 culus Ficaria, this explanation is untenable. He employed the 

 klinostat or slowly-rotating apparatus of Sachs (which, by the 

 way, was originally invented by John Hunter), by which means 

 the influence of gravitation is destroyed. And he found that 

 even under these circumstances, when apogeotropism could not 

 come into the problem, the leaves were able to adjust themselves 

 so as to be at right angles to the light. Here there can be 

 no question of a balance between opposing tendencies, since the 

 important factor of apogeotropism is removed ; it is therefore 

 clear that Frank's views are strongly supported by these experi- 

 ments ; and we must for the present accept the term diaheliotro- 

 pism. Such a term is merely a convenient label for a certain 

 region of ignorance, and should stimulate rather than hinder 

 further research into the subject. 



Another paper by Mr. F. Darwin, on Climbing Plants, was pub- 

 lished in a popular form in the journal ' Kosmos ;' whilst his re- 

 mai'ks on "Circumnutation in certain one-celled Organs" came out 

 in the ' Botanische Zeitung ' last July. A paper by Mr. Shattock 

 was also read before the Society, on the reparative processes which 

 occur in vegetable tissues. The author's investigations had been 

 prosecuted without knowing of l^j-ank's researches on the same 

 subject, but were confirmatory of the latter. Herr Schwarz has 

 written an article on the influence of gravity on the growth 

 lengthwise of plants, which appeared in the Tiibingen volume 

 edited by Dr. W. Pfefter. Herr Krasan has been investigating 



