MORPHOLOGY 



131 



The cambium cells have, for the most part, the shape of right-angled prisms, of 

 which the radial diameter is smaller than the tangential. The ends of these prisms 

 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 

 mi'ilullary rays are formed are the 

 shorter. The primary vascular por- 

 tions of the bundles projecting into 

 the medulla constitute what is known 



as the MEDULLARY SHEATH. 



Owing to climatic varia- 

 tions, the cambial tissue of 

 woody plants exhibits a period- 

 ical activity which is expressed 

 by the formation of ANNUAL 

 KINGS of growth (Figs. 139, 

 141 A, 146). In spring, when 

 new shoots are being formed, 

 wider tracheal elements are 

 developed than in the follow- 

 ing seasons (Figs. 141 A, 148). 

 For this reason a difference is 

 perceptible between the EARLY 

 WOOD (spring wood), which is 

 composed of large elements 

 especially active in the convey- 

 ance of water (Fig. 141 /), 

 and the LATE WOOD (autumn 

 wood), consisting of narrow 

 elements which impart to a 

 stem its necessary rigidity 

 (Fig. 141 s}. Throughout the 

 greater part of the temperate 

 zone, the formation of wood 

 ceases in the latter part of 



AugUSt Until the following ,, l(i uo ._ 1Magnml to illuslral( . the secondary growth 



-spring, when the larger ele- inthickneMCrftheitemaadvQofcofaGyBmoapenn 



mentS of the Spring WOod are or Dicotyledon. , Cambium, indicated by; 11 dotted 



, ! j f* . line ; m, pith ; 1, 2, 3, 4, successive animal rings 



again developed. Owing to , jf W01K , . ,, h . lst . S( , % limit ,,, UW)I Imiin stem _,,, 



the contrast in the struc- tap-r..t. 



ture of the spring and the 



autumn wood, the limits (Fig. 139 i) between successive annual 



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



to the naked eye, and so serve as a means of computing the age 



of a plant. 



Under certain conditions tin- number of annual rings may exceed the number 

 of years of growth, MS. for instance, when MIDSUMMKII cii'wni occurs, such as 



