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PART II. ANATOMY AND HISTOLOGY. 



[35 



later in the year. The anatomical cause of the distinctness of 

 the annual rings is the same in all cases, namely, that thejast- 

 formed xylem-elements of an annual ring have a very small radial 

 diameter as compared with those formed when growth is resumed 

 in the following spring. In Conifers this distinction is emphasized 

 by the fact that the spring-wood is formed of thin-walled 

 tracheids (Fig. 113 /") and the autumn-wood of smaller thick- 

 walled tracheids (Fig. 113 fi). In dicotyledonous trees the num- 

 ber 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 



FIG. 112. Part of a transverse 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- 

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



FIG. 113. Transverse section of por- 

 tion of the secondary wood of a branch 

 of the Fir at the junction of two annual 

 rings : m a medullary ray all the other 

 cells belong to the wood ; / large-celled 

 spring- wood ; ih small-celled autumn- 

 wood; w the limit between the autumn- 

 wood of one year and the spring-wood 

 of the following year ; between h and w 

 is the flattened limiting layer ( x 250) . 



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). 



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 distribu- 

 tion of the constituents of the wood. 



The secondary wood gradually becomes distinguishable into an 



