THE TREE ITS SHOOT-SYSTEM, 



105 



&\ 



p.v. 



sp. 



FIG. 28. The various chief elements of the wood of the oak, isolated by 

 maceration, and highly magnified : y, a fiber, distinguished by its 

 thick walls, simple slit-like pits, and no contents ; w.p, part of a row 

 of wood-parenchyma cells, with simple pits, and containing starch in 

 winter ; tr, a tracheid, distinguished from the fiber especially by its 

 bordered pits ; p.v, part of a rather large pitted vessel, made up of 

 communicating segments, each of which corresponds to a tracheid, 

 and has bordered pits on its walls ; sp, part of a spiral vessel. 



