ANATOMY OF VEGETABLES. 105 



mate. Every year's growth constitutes a layer, so 

 that by counting the several layers the age of the tree 

 may be ascertained. 



In a species of ash these layers are very distinct, 

 and are so slightly connected by the cellular substance 

 between them, that by continual beating and bending, 

 the basket-maker is enabled to separate them for his 

 use. These layers are annually secreted or deposited 

 from the innermost part of the bark or liber. The wood 

 owes its tenacity and strength to innumerable woody 

 fibres, and consists of various vessels running for the 

 most part longitudinally, PI. 3, fig. 17, b. Some of 

 these vessels have a spiral coat, others not. Some have 

 their 9 ides perforated, and others nearly or quite en- 

 tire see pi. 3, fig. 19 & 20. The vessels anastomose 

 or communicate laterally with each other. This is 

 the case also with the vessels of the bark, which are 

 smaller and more complicated thaw those of the wood. 



Observation. In the transverse sections of wood, which are a 

 common and beautiful object in the microscope, these vessels are 

 very conspicuous. In the roots of the Elm, the trunk of the Com- 

 mon Creeper, and the wood of the Chesnut, they are so large that 

 air and even water can be sucked through them in pieces of sever- 

 al feet in length. 



Of the vessels of the wood, some in their youngest 

 state convey the sap from the root to the extremities of 

 the branches and leaves, and others contain the vari- 

 ous peculiar or secreted juices. The whole are joined 

 together by the cellular substance, which seems to be 

 designed to unite all the parts of a plant. 



In some trees, and particularly old ones, there are a 

 number of external layers or grains of wood which 

 differ greatly in colour from the central ones, being 

 much whiter ; these are called by the workmen the sap, 

 while the central or darker part is called the heart. 

 The sapwood is called technically alburnum. 

 - The sapwood is the living, vascular, and active part, 

 while the heart is dead or possesses a very low degree 



