CHAP. I.] 



PLANT ARCHITECTURE. 



49 



composed of vessels with very large cavities, gradually 

 changes towards the autumn into very dense wood, with 

 thick cell- walls and very minute cavities. Each season 

 the porous spring wood following abruptly on the dense 

 wood of the preceding autumn causes the rings known 

 as annual rings. The following are the most important 

 types of cell structure forming xylem and phloem respec- 

 tively : 



Xylem. There are two distinct types of cell structure; 



P. L. P. I. P .L.MC. C . F . V . F , V.F.V ,T.M, 



Fig. 16. Horizontal section of a Dicotyledon, the maple, showing 

 three years' growth ; S, cortex ; M, pith ; C, cambium ; the figures 

 i, 2, 3, indicate the three annual rings of wood, including the cam- 

 bium ; the dark portion between the cambium and the cortex is the 

 bast or phloem. (Magnified. ) 



wood cells or tracheides, consisting of long, thick-walled 

 cells with tapering, pointed ends that overlap, the walls 

 become hard and rigid, and form the great bulk of 

 durable wood. Water passes from the roots to the 

 leaves through the substance of the cell- walls of the 

 youngest and last-formed wood-cells. Tracheides are 

 formed in most instances directly from single cambium 

 cells ; vessels are recognized by their large diameter 



