[62 



LECTURE X. 



on the transverse section as rings ; and, since one of these is formed each 

 year, they are distinguished as annual rings. These are, of course, in reahty hollow 

 cylinders disposed one around the other. Each annual ring is the product of the 

 wood-forming activity of the cambium-ring during one period of vegetation. That 

 these appear, even to the unaided eye, as layers of the wood-body sharply separated 

 from one another, depends upon the fact that within any one period of vegetation 

 (i.e. in the time during which an annual ring is formed), the formation of wood itself 

 varies periodically. It is evident that the inner portion of any one annual ring is the 

 first to be developed in the period of vegetation — that is, it is formed in the spring ; 

 while the outer portion of the same annual ring has arisen towards the conclusion of 

 the wood-forming activity in the same period of vegetation. At the boundary of suc- 

 cessive annual rings, therefore, 

 the spring-wood of the follow- 

 ing ring is always deposited 

 on the autumnal woo i of the 

 preceding next inner ring ; 

 so that the usually different 

 wood formations of two pe- 

 riods of vegetation lie imme- 

 diately adjoining one another. 

 Owing to this, the separation 

 of two neighbouring annual 

 rings is more clearly marked. 

 Put quite generally, the spring- 

 wood is composed of ele- 

 ments with wide lumina and 

 thin walls ; the autumn wood 

 last formed in a period of ve- 

 getation consists, on the con- 

 trary, of elements with narrow 

 lumina, and often also thick 

 walls. This difference comes 

 out very clearly in the simple 

 wood of the Conifers, which 

 consists, as stated, entirely of tracheVdes. The transverse sections of these are nearly 

 square in the spring wood ; in the autumn wood, on the contrary, they are narrow 

 and rectangular, so that the cells appear compressed in the radial direction to 

 one-third or one-fourth the diameter of those of the spring wood. 



Apart from the stronger thickening of the walls, the autumn wood is al^o 

 richer in wall-substance and poorer in cavities than the spring wood. In the 

 dictyledonous woods there is associated with these differences affecting the paren- 

 chyma, tracheides, and libriform fibres, a corresponding difference in the vessels, 

 so that in general the vessels l}-ing in the spring-wood are far larger in trans^•erse 

 section than those formed in the summer and autumn. Frequently, the relative 

 number of vessels in the rings of wood also diminishes with the advancing season. 

 Thus, taken all together, the spring wood is more porous, looser, and poorer in 



FIG. i68.— Transverse section of the wood of Rhainnns fj 

 autumn-wood of an older annual ring ; w vessels in the spring-woe 

 ring (after Rossmann). 



