130 



BOTANY 



active division. \\ 7 ithin the bundle may be seen two large vessels 

 (m"), in a still incomplete state ; while in the adjoining primary 

 medullary rays the cells which give rise to the interfascicular 

 cambium may still be plainly distinguished. All the tissue arising 

 from the inner side of the cambium ring goes to form the secondary 

 WOOD, while that produced on the outside is termed secondary 

 BAST. The vascular portions of the wood form the WOOD STRANDS, 

 the sieve portions within the bast the BAST STRANDS. By the 

 activity of the interfascicular cambium, the primary medullary 



s 



Fir,. 139. Portion of a four-year-ol<l stem ot tin- 1'inc, 1'innxlsi/lcexti-i*, cut in winter. /, Trans- 

 verse view ; /, 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 ; ma, ins', ?.<"', medullary rays in transverse, radial, and tangential view ; 

 ins", radial view of medullary rays in the bast ; c, cambium ring ; b, bust ; //. n-sin canals ; In; 

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



rays are continued throughout both the wood and bast. As the 

 wood and bast strands enlarge, SECONDARY MEDULLARY RAYS are 

 developed from the 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 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 transvcrx- parenchyma is also 

 sometimes used to designate the medullary rays, winch arc in fact composed almost 

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

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

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



