SECT. I 



MORPHOLOGY 



123 



are usually one-sided, tapering to a point, alternately on the right and left sides. The 

 length of the cambium cells varies in different plants, but those from which medul- 

 lary rays are formed are the shorter. The primary vascular portions of the bundles 

 projecting into the medulla constitute what is known as the medullary sheath. 



Owing to climatic variations, the cambium tissue of woody plants 

 exhibits a periodical activity which results in the formation of 

 annual rings of growth (Figs. 137, 139, 145). In spring, during the 

 period of energetic growth, larger tracheal elements are developed 

 than in the following seasons (Figs. 139, 146). For this reason a 



Fig. 137. — Portion of a four-year-old stem of the Pine, Pinus sylvestris, cut in winter, q, Trans- 

 verse view ; I, radial view ; t, tangential view ; /, early wood ; s, late wood ; m, medulla ; p, 

 protoxylem ; 1, 2, 3, 4, the four successive annual rings of the wood ; i, junction of the wood 

 of successive years ; ms, ms 1 , ms"', medullary rays in transverse, radial, and tangential view ; 

 Tits", radial view of medullary rays in the bast ; c, cambium ring ; &, bast ; h, resin canals ; Zjr, 

 bark external to the first periderm layer, corresponding to the primary cortex. ( x (3.) 



difference is perceptible between the early WOOD (spriDg wood), which 

 is composed of large elements especially, active in the conveyance 

 of water (Fig. 139,/), and the late wood (autumn wood), consisting 

 of narrow elements which impart to a stem its necessary rigidity (Fig. 

 139, s). Throughout the greater part of the temperate zone, the forma- 

 tion of wood ceases in the latter part of August, until the following 

 spring, when the larger elements of the spring wood are again 

 developed. Through the consequent contrast in the structure of the 

 early and the late wood, the limits (Fig. 137, i) between successive 

 annual rings of growth become so sharply defined as to be visible 



