BOOK in.] Introduction. 361 



no circulation of sap in plants corresponding to the circulation 

 of blood in animals, the result was obtained by the aid of this 

 hypothesis derived from a comparison between animals and 

 plants. The important discovery that leaves play a consider- 

 able part in the nourishment of plants, was to some extent an 

 incidental product of the investigation of the former question, 

 and it preceded that of the decomposition of carbon dioxide 

 by the green parts of plants by more than a hundred years. 

 To give another example ; it was obviously a comparison 

 of certain phenomena in vegetable life with the propagation of 

 animals which paved the way for the discovery of sexuality 

 in plants ; long before Rudolf Jacob Camerarius made his 

 decisive experiments (1691-1694) on the necessary co-opera- 

 tion of the pollen in the production of seeds capable of 

 germination, the idea had been entertained that there might 

 be an arrangement in plants corresponding to the sexual re- 

 lation in animals, though that idea was highly indistinct and 

 distorted by various prepossessions. In like manner the interest 

 excited by the discovery of the irritability of the Mimosae in 

 the i yth century, and of similar phenomena of movement in 

 plants at a later time, was mainly due to the striking resem- 

 blance suggested between animals and plants ; and the first 

 researches into the subject were obviously intended to answer 

 the question whether the movements in plants are due to con- 

 ditions of organisation similar to those in animals. In all 

 cases of this kind it was matter of indifference whether 

 the analogies presupposed were finally confirmed after pro- 

 longed investigation, as in the question of sexuality, or dis- 

 proved as in that of the circulation of the sap. The result 

 was of less importance than the obtaining points of departure 

 for the investigation. It answered this purpose to adopt cer- 

 tain actual or only apparent analogies between plants and 

 animals, and to assume, to some extent to invent, certain 

 functions for the apparently inactive organs of plants, and 

 to interrogate them upon the point. Scientific activity was 



