198 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[35. 



fact that the spring- wood is formed of thin- walled tracheids (Fig. 

 150 /) and the autumn-wood of thick- walled tracheids (Fig. 150 ft). 

 ]n dicotyledonous trees the number and size of the vessels 

 diminishes in each annual ring from its inner to its outer limit. 

 When this takes place very gradually, the eye cannot detect any 

 conspicuous difference between the spring- and autumn-wood (as 

 in the wood of the Beech, Lime, Maple, and Walnut) ; but some 

 kinds of wood show a ring of conspicuously large vessels in the 

 spring- wood, while in the autumn- wood there are numerous much 

 smaller vessels (as in the wood of the Oak, Elm, and Ash). 



FIG. 149. Part on a iransverse section of a 

 twig of the Lime, four years old (slightly 

 magnified): m pith; ms medullary sheath; x 

 secondary wood ; 1 2 3 4 annual rings; c cam- 

 bium ; pa dilated outer ends of primary medul- 

 lary rays ; b bast ; pr primary cortex ; fc cork. 



FIG. 150. Transverse section of por- 

 tion of the secondary wood of a branch 

 of the Fir at the junction of two annual 

 rirgs : ra a medullary ray all the other 

 ( ells belong to the wood ; / large-celled 

 spring- wood; 7i small-celled autumn- 

 wood ; w the limit between the autumn- 

 wood of one year and the spring-wood 

 of the following year; between 7i and w 

 is the flattened limiting layer (x 250). 



The thickness of the annual ring varies in different plants, and 

 even in any one plant, under different conditions of growth ; and 

 not only the thickness, but also the number and relative distri- 

 bution of the constituents of the wood. 



The following case will serve to illustrate the variation in thickness and 

 structure of the annual ring. In a well-grown Ash-tree (Fraximts excelsior] 

 the annual ring was found to be 2-3 mm. in thickness, and to consist of an in- 

 ternal (spring) zone of wide vessels with wood-parenchyma and rather thin- 

 walled woody fibres, followed by a layer of thick-walled woody fibres with 

 scattered smaller vessels surrounded by wood-parenchyma, and then by an ex- 

 ternal (autumn) zone consisting of wood-parenchyma with small very thick- 

 walled vessels. lu vigorous young Ash-trees growing in a damp soil, the annual 



