The Structure of a Tree 



water flows up to the leaves. In the cut end of a grape- 

 vine, in the root of a willow, and in many woods these 

 large vessels are plainly visible 

 as open pores. Is it possible 

 to draw air through them? 



Others of the cells become 

 changed into tracheids. These 

 have thicker walls than the 

 tracheae, and the end walls 

 are not absorbed. They do 

 not, therefore, form long open 

 vessels in the tree, and the 

 water makes its way from 

 tracheid to tracheid by pass- 

 ing through thin places in 



the Walls. The tracheids FIG. 8. Tracheids from a basswood 

 Conduct Water more Slowly tree (^) and wood fibers from a 



J hard maple (5). 



than do the much larger 



tracheae that have no cross walls, but they give more 



strength to the wood. 



A third element that is prominent in wood, especially 

 in hard woods, is the wood fibers. These are formed 

 by groups of cells elongating and forming slender fibers 

 with pointed ends and thick walls. These fibers are 

 much like the tracheids, but they are smaller and have 

 walls so thick that the spaces within them are some- 

 times almost closed. They do not conduct water, but 

 when the protoplasm in them dies they stand in a tree 

 like bundles of little, tightly sealed tubes filled with air. 

 The function of the wood fibers is to give strength to 

 the wood ; their thick walls make it difficult to crush 



