PRINCIPLES OF VEGETATION. 



349 



the whole of this parenchyma ; but the Jargefl ones ftand on the inner 

 edge of the rind, and the outer edge of the pith, juft at the two extre- 

 mities of each lamellse. 



The fecond kind of parenchyma is placed between the rind and the 

 pith, is divided into diftindt bodies by the lamellae; and each ofthefe 

 bodies forms a large bag. 



Thcfe bags contain a third parenchyma, which is a clufter of fmalter 

 bags, diftinfl and unconnefled with each other, having a fmall flalk by 

 •which they are fixed to the large bag. Within each of thefe fmall bags 

 are many hundreds of bladders, compofed of extremely minute fibres. 

 Thefe bladders contain the acid juice of the lemon. 



The vefTels in the roots of plants abforb ir.oifture from the earth, and 

 convey it to the trunk, branches, leaves, &c. This moifture, when it 

 firft enters the root, is crude ; but as it alcends into the other ^arts of 

 the plant, it undergoes feveral changes, by means of the different configu- 

 rations of the veflels peculiar to each part. Thus the leaves, flowers, 

 fruit, and feed, have all fomething peculiar in the ftrudure and arrange- 

 ment of their veflels, which produces conflderable changes in the nature 

 of the fap. It is not known how thefe changes are produced : but how 

 the ftomachs of animals make chyle from animal and vegetable fub- 

 ftances, or how urine, faliva, bile, &c. is fecreted from the common mafs 

 of blood, is as little known. The fap likewife moves in a lateral or ho- 

 rizontal direction. 



Philofophers were greatly divided about the circulation of the fap. 

 Some contended that it returns to the root betwixt the bark and wood. 

 But Hales, who has made many accurate experiments on the fubjeft, has 

 Ihown, that it does not circulate, but afcends and defcends in the fame 

 veflTels ; that it alcends in hot weather, and defcends in cold, like the 

 ijairits in a thermometer. 



Vegetables begin to abforb fap about the beginning of fpring, and 

 foon after ihoot out their buds, leaves, and flowers. 



When plants are in a ftate of vegetation, efpecially in hot weather, 

 there is a great deal of fuperfluous fapabforbed; but the fuperfluity is 

 carried off by tranfpiration. Every part of a plant tranfpires, but the 

 greateft quantity paffes by the leaves. 



Some have afligned the tranfpiration of plants as the caufc of the mo- 

 tion of the fap. It is undoubtedly one caufe of this motion; becaufc, if 

 the tranfpiration be flopped, at a time when all the veffcls are full, the 

 motion of the fap muft ftop of courfe. But then there is a previous and 



3 N 2 principal 



