BOOK in.] Introduction. 375 



less critical sifting in Unger's text-book of 1855 ; these were 

 the two books which did most to disseminate a knowledge 

 of the subject up to 1 860, and they performed their task with 

 credit ; that which appears in Schacht's books after 1852 under 

 the head of vegetable physiology rests on such imperfect 

 acquaintance with this branch of science, as to diminish rather 

 than increase its reputation. 



Passing from this preliminary survey to a more detailed 

 account of the subject, it will be found necessary to keep 

 the history of the sexual theory distinct from other questions 

 in vegetable physiology. This mode of proceeding is required 

 by the fact, that the establishment and further elucidation 

 of the decisive points in the sexual theory were made inde- 

 pendently of the rest of physiology, so that the historical 

 continuity would be interrupted and the account rendered 

 obscure by any attempt to connect the development of the 

 theory chronologically with other topics. In like manner the 

 doctrine of the nutrition of plants and of the movement of the 

 sap was developed uninterruptedly and in independence of 

 other physiological matters ; it will be advisable therefore to 

 devote a separate chapter to those subjects also. Earlier 

 discoveries respecting the movements of the parts of plants and 

 the mechanics of growth will be briefly recounted in a third 

 chapter. 



