360 Introduction. [Book hi. 



animals were known to every one, at least in their more 

 obvious features, the study of vegetable life had to begin with 

 laborious enquiries, whether the different parts of plants are 

 generally necessary to their maintenance and propagation, 

 and what functions must be ascribed to individual parts for 

 the good of the whole. It was no easy matter to make the 

 first step in advance in this subject ; something can be learnt 

 of the functions of the parts of animals from direct observa- 

 tion, scarcely anything in the case of plants ; and it is only 

 necessary to read Cesalpino and the herbals of the 16th 

 century to see how helpless the botanists were in every case 

 in presence of questions concerning the possible physiological 

 meaning of vegetable organs, when they ventured beyond the 

 conceptions of the root as the organ of nourishment, and 

 of the fruit and seeds as the supposed ultimate object of 

 vegetable life. The physiological arrangements in vegetable 

 organs are not obvious to the eye ; they must be concluded 

 from certain incidental circumstances, or logically deduced 

 from the result of experiments. But experiment presupposes 

 the proposing a definite question resting on a hypothesis ; 

 and questions and hypotheses can only arise from previous 

 knowledge. An early attempt to connect the subject with 

 existing knowledge was made in the use of the comparison 

 of vegetable with animal life, a comparison which Aristotle 

 had employed with small success. Cesalpino, provided with 

 more botanical and zoological knowledge, endeavoured to 

 arrive at more definite ideas of the movement of the nutrient 

 juices in plants, and when Harvey discovered the circulation 

 of the blood in the beginning of the 17th century, the idea at 

 once arose that there might be a similar circulation of the sap 

 in plants. Thus a first hypothesis, a definite question was 

 framed, and attempts were made to decide it by more exact 

 observation of the ordinary phenomena of vegetation, and 

 still better by experiment ; and though a discussion which 

 lasted nearly a hundred years led to the opinion that there is 



