146 



BOTANY 



PART I 



the interfascicular cambium in great part gives rise to tissues similar 

 to those formed by the fascicular cambium. Thus, in place of the 

 original broad medullary rays, the cambium forms at definite points 

 narrower radial rows of medullary ray tissue. These medullary rays, 

 which are spindle-shaped when cut across (Fig. 170), traverse the 

 wood and the bast, connecting the pith with the cortex as PRIMARY 

 MEDULLARY RAYS. As the thickness of the secondary wood and bast 

 increases, SECONDARY MEDULLARY RAYS are developed from the 

 fascicular cambium. In one direction the secondary medullary rays 



FIG. 169. Diagrammatic 

 figure of the shape of 

 cambial cells. A, I and 

 II, the two forms 

 which occur, seen from 

 the tangential face ; B, 

 in radial section ; C, 

 in transverse section. 

 (After ROTHERT.) 



FIG. 170. A diagrammatic tangential 

 section to illustrate the subdivi- 

 sion of a primary medullary ray 

 into many smaller rays on the 

 commencement of secondary thick- 

 ening. I, I; Adjoining primary vas- 

 cular bundles ; pm, primary medul- 

 lary ray transformed by the 

 activity of the interfascicular cam- 

 bium into many small spindle- 

 shaped medullary rays and reticu- 

 lately - connected secondary vas- 

 cular bundles. 



end 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 (Fig. 179). 



The cambial cells which give rise to medullary rays are shorter and 

 their end walls are more horizontal, for when a medullary ray is to be 

 initiated the ordinary cambium cell becomes divided transversely or 

 obliquely. 



The origin of the cambium and the nature of its activity can be distinguished 

 into three main types according to the primary construction of the stem : 

 1. The stem has a circle of collateral vascular bundles separated from one 

 another by broad medullary rays ; the breadth of % the medullary rays is main- 

 tained during secondary growth, the interfascicular cambium producing only 

 medullary ray tissue. This is thecase for many herbaceous plants, but among 



