676 Microscopical Essays. 



the branches, the leaves, the flower, and the fruit. The com- 

 ponent parts of thefe divifions are not complicated, they are 

 fimple when compared to thofe of an animal, and this becaufe the 

 offices of the vegetable are fewer than thofe of the animal. 



The interior part may be confidered as confifting of lignous 

 fibres, interfperfed with a vail number of bladders, which are 

 here named the cellular \iffue, the vafa propria, and the lap 

 velfels ; though thefe are confidered by fome writers as mere air 

 velfels. 



The lignous fibres are very fine tubes, proceeding nearly in a 

 vertical direction from the top to the bottom of the tree ; they 

 are fometimes parallel to each other, fometimes they divaricate, 

 and often leave oblong intervals or Ipaces. There is great reafon 

 for fuppofing them to be a fpecies of lymphatic velfels. The 

 vacant fpaces between thefe fibres are filled up by a veficular 

 membrane, lying in an horizontal direction, and which is called 

 in this chapter the cellular tifTue. 



The vafa propria are formed of lignous fibres, but differ from 

 the foregoing in their fize, and in the juices which they contain. 

 In the part properly called the wood, we meet with the Tap 

 velfels ; but as in fome Hates they feem as if they were formed of 

 a filver-coloured fpiral membrane, and are found without any 

 juices, they have been fuppofed to be air velfels, and called the 

 trachea, making up an arterial fyftem, and fupplying the place of 

 the heart in animals. 



The 



