SECT. I 



MORPHOLOGY 



HI 



fascicular cambium. In one direction the secondary medullary rays 

 terminate blindly in the wood, and in the other in the bast ; the later 

 they develop, the less deeply do they penetrate the tissues on either 

 side of the cambium. 



The primary medullary rays are therefore often distinguished as long, the 

 secondary as short medullary rays. The expression transverse parenchyma is also 

 sometimes used to designate the medullary rays, -which are in fact composed almost 

 exclusively of parenchymatous tissue. The cells given off by the initial layer of the 

 cambium for the formation of medullary rays do not undergo a further tangential 

 division, as in other cases, but assume at once the character of medullary ray cells. 



Fig. 149.— Portion of a four-year-old stem of the Pine, Piiitts sijlvestris, cut in winter, q, Trans- 

 verse view ; I, radial view ; t, tangential view ; /, spring wood ; s, autumn 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'", ms, medullary rays in transverse, radial, and tangential view ; 

 vis", radial view of medullary rays in the bast ; c, cambium ring ; b, bast ; h, resin canals ; br, 

 bark external to the first periderm layer, and formed from the primary cortex, (x 6.) 



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. Those from which 

 medullary rays are formed are shorter than the other cambial cells. The primary 

 vascular portions of the bundles i)rojecting into the medulla constitute what is 

 known as the medullary sheath. 



Owing to climatic variations, the cambial tissue of woody plants 

 exhibits a periodical activity which is expressed by the formation of 

 ANNUAL RINGH of growth (Figs. U9, 151 J, 1.55). In spring, when 

 new shoots are being formeu, wider tracheal elements are developed 



