170 SCIENCE PROGRESS. 



this consoled him for the fact that the botanists, now as on 

 other occasions, instead of testing the innovation in its 

 general application, sought only too zealously for instances 

 in which it did not apply. But the time will surely come 

 when it will be deemed absurd to describe a Caulerpa, for 

 instance, as a " unicellular" plant, and it fell to Sachs to fit 

 scientific nomenclature to recent advances in knowledge. 

 It was self-evident to him that definitions are only a means 

 towards generalisation and that they have absolutely no 

 validity in themselves. 



The essay upon orthotropic and plagiotropic plant- 

 parts takes us into a region that lay nearest to Sachs' heart 

 during the last years of his life, namely, that of physiologi- 

 cal or causative morphology. In this treatise he deals with 

 the connection between the structure (in the widest sense 

 of the word) and the direction of the organs. The defini- 

 tions "orthotropic" and "plagiotropic" were introduced, 

 and referred more particularly to the dorsiventral structures 

 that had long been neglected under the supremacy of the 

 "spiral theory". He does not merely treat of the purely 

 structural conditions, but of the causative relations between 

 orthotropic growth and dorsiventral structure. Sachs would, 

 I believe, have altered later his theoretical conclusions 

 upon plagiotropism ; they are based upon ideas which he no 

 longer held, as we may see in the text, to be as thoroughly 

 warranted as formerly. But putting aside these points, 

 about which opinions still differ, we find ideas in this essay 

 that are still working with considerable effect in morphology. 



As a morphologist Sachs' activity displayed itself in one 

 direction by some special studies that date from his earlier 

 years, in another by his text-books, and again by his final 

 general essays. 



His two treatises on Collema 1 and Crucibulum, show 

 him at work in the region of cryptogams. It was he who 

 in his Text-book defended Schwendener's Lichen theory 



1 In this essay he approached very closely to the later lichen theory, 

 when he said that it looked as if a parasitical fungus had established itself 

 in the nostoc ; he believed that the nostoc-heterocysts might develop into 

 a mycellium. 



