662 REPORT— 1891. 



along with sucli 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. Diitrochet's clear statement (1824) as to growth-curvatures heing au 

 affair of stimulus and response wiU be quoted lower down. Treviranus in his 

 ' Physiologie '(1838) speaks of geotropism as a Trieb, or impulse, and adds that 

 thougli 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 Ilofmeister 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 explana- 

 tion to acellular and multicellular organisms. But it led hini 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. 



Irritahility. 



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 irritahility to the phenomenon, and we 

 call the influence which produced the cbange a stimulus. At this time (1866) 

 there was, as far as I can discover, no idea that growth-curvatures were pro- 

 duced by external influences acting as stimuli. Gravitation and light were sup- 

 posed 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 descendant des racines et de I'ascension des tiges ; 

 elle n'en est point la cause immediate ; elle agit dans cette circonstance comme 

 agent nervimoteur. Nous verrons plus has de nouvelles preuves de la gen^ralitg 



iprimes par cet agent, 

 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 'Beitriige zur Pflanzen- 

 physiologie,' which was of importance in more than one way. In this work the 

 term ' geotropism ' was first suggested in imitation of the existing expression ' helio- 

 tropism.' This uniformity of nomenclature had an advantage beyond mere con- 

 venience, 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 



' Recherches anat. svr la Sfrvctttre intime, §-c. 1824, p. 107. Dutrochet, however, 

 was not consistent in this matter, and later on gave explanations as mechanical a& 

 Hofmeister's. 



