56 ON VEGETABLE PHYSIOLOGY. A 



To consider vegetable physiology in all its re- 

 lative bearings, it will be requisite to divide the 

 subject into three distinct heads. Firstly, the 

 structure and functions of those parts of a vege- 

 table which contribute to its nourishment and 

 preservation, including the root, trunk, branches, 

 and leaves. Secondly, of those organs which are 

 necessary to its re-production, and these have 

 reference to the flower, fruit, and seed. And 

 lastly, the process of re-production, embracing 

 the structure of the seed, its germination, and the 

 formation and completion of a new plant. 



But previously to entering into a particular 

 explanation of the several organs, it will be proper 

 to inform you that the general structure of vegeta- 

 bles consists originally of only two parts, the 

 fibrous portion and the cellular tissue: all the other 

 materials are derived from extraneous sources, 

 and therefore are not considered as belonging to 

 their structure. It is the arrangement and modi- 

 fication of the above two substances, which alone 

 constitute the difference in the formation of each 

 part of the plant. 



The fibrous portion, it has been ascertained, 

 consists of linear and spiral tubes, or vessels, 

 which, in vast numbers take various directions 

 through every part of the plant, and form the 

 channels of circulation for its respective fluids. 

 So long as these vessels remain pervious, they 

 continue tubular ; but when trees become large 



