Lecture XXII. 187 



pith to the cortex, which are the primary rays, and persist from 

 the primary arrangement of tissues obtaining before the differenti- 

 ation of interfascicular cambium. The secondary medullary rays 

 of Pinus are usually only a single layer of cells thick and from one 

 to ten, or even fifteen, cells deep. The interfascicular cambium 

 forms woody tubes in the primary medullary rays. The primary rays 

 are thus reduced in thickness, as they extend outwards, keeping pace 

 with the thickening of the wood, and soon become, like the 

 secondary rays, only one cell thick. , The reduction in the size of 

 the primary rays by the addition of secondary wood converts the 

 separate woody strands, such as we have seen are found near the 

 apex, into a continuous woody cylinder a little lower down. This 

 change is externally visible in the greater rigidity and hardness of 

 the stem at this level, compared with its succulent and herbaceous 

 consistency nearer to the tip. 



A considerable amount of secondary wood may be formed by 

 the close of the first growing season. Thus, if at the end of the 

 summer we cut across a tip developed during the previous spring, 

 we will find the woody cylinder already consolidated into a 

 rigid tube. The primary wood forms ridges running down the 

 inside of this tube and the protoxylem threads occupy the crests 

 of these ridges. The position of the protoxylem on the inner 

 aspect of the woody strands is characteristic of the wood of the 

 stems of Seed Plants. Such wood strands are called endarch and 

 contrast with those of the Ferns in which the protoxylem is often 

 surrounded by, and embedded in, the woody strand (mesarch). 



As the season progresses the woody tubes or tracheids formed 

 by the cambium are narrower than those formed earlier in the 

 season. At the same time their walls are thicker. At the end of 

 the summer the activity of the cambium ceases. Cell division is 

 suspended and no differentiation of cambial cells into tracheids 

 and sieve-tubes takes place. However, with the return of spring 

 growth recommences. The cambium-cells grow in thickness and 

 they divide longitudinally. At the same time differentiation begins 

 among the cells forming its inner and outer layers. The former 

 become tracheids and the latter sieve-tubes. The tracheids which 

 are formed in the spring are much wider than those formed in the 

 summer and their walls are thinner. The cambium adds tracheid 

 after tracheid to the wood, and as the season advances the radial 

 diameter of the tracheids successively added becomes gradually 

 smaller and their walls thicker. In this way the layer of wood 

 formed during each growing season gradually changes in character ; 

 that formed in the spring is built of wide tubes with thin walls, 



