144 



THE ANATOMY OF WOODY PLANTS 



In the case of a conifer, for example, the root is clearly distinct from 

 the stem by the possession of a woody cylinder devoid of pith. The 

 primary wood, moreover, in the root is very well developed and 

 forms a lens-shaped mass in the center of the cylinder which is 

 clearly centripetal in its development, since the smaller elements 

 are situated in two or more groups hi an external position. The 



primitive organization of 

 the root in contrast to the 

 more progressive organi- 

 zation of the stem is par- 

 ticularly well indicated in 

 connection with the rays. 

 In the cauline woody cyl- 

 inder these appear to take 

 their origin from the pith; 

 hence arises the com- 

 monly employed appella- 

 tion, medullary rays. The 

 intimate relation between 

 rays and pith which appa- 

 rently obtains in this case 

 is in reality merely a sem- 

 blance, owing to the fact 

 that the primary wood 

 has become nearly obso- 

 lete hi stem organs in the higher gymnosperms and the angio- 

 sperms. In the root the primitive situation in which the radial 

 parenchyma has no relation whatever to medullary tissues is very 

 clear. It is obvious that the common term medullary rays is 

 extremely inappropriate as applied to the radial parenchyma of 

 the secondary wood. The true situation is not only revealed by 

 the comparison of stem and root, but it also becomes even more 

 apparent when the stem in older types is compared with that 

 found in existing forms. The more detailed consideration of the 

 contrasts in organization between stem and root is best deferred 

 to later pages, after the anatomy of the various groups has been 

 discussed. 



FIG. 101. Transverse section of the upright 

 stem of Lycopodiiim clavatum. 



