CONIFERALES 241 



lem from mesarch to exarch. It will be remembered that, in the 

 cycads, bundles which are endarch in the stem become exarch in the 

 leaves. Cotyledons and cone axes are regarded as conservative 

 structures, and consequently more likely to retain ancient charac- 

 ters. Stiles^^' finds some centripetal xylem in the axis of the male 

 cone of Saxogothea. Cotyledons, cone axes, and young seedlings 

 would probably yield many cases of mesarch structure. 



In the other four families no distinct mesarch character is evident, 

 although there may be traces of it in Cuprcssus and Juniperus and 

 in the ovuliferous scales of Araucaria. Buchholz^^ has recently 

 made a very careful examination of the cotyledons of Ccdrus, but 

 found no centripetal xylem, a conclusion in accordance with the 

 findings of Hill and de Fraine for other members of the Abietaceae. 

 Of course, if transfusion tissue is interpreted as xylem, the needles of 

 Pinus and many others would have mesarch bundles. 



The protoxylem, as in all vascular plants except the angiosperms, 

 consists entirely of tracheids, there being no continuous vessels 

 formed by the breaking down of the end walls of adjacent cells. The 

 first cells of the strand to become lignified may be annular, but the 

 succeeding ones have the spiral marking. 



The secondary wood consists of tracheids traversed by medullary 

 rays. A detail of the structure of the secondary wood of a typical 

 conifer is shown in fig. 248. 



In this figure the large, comparatively thin- walled tracheids of 

 the spring wood are shown at the right; and the smaller, thicker- 

 walled cells of the later wood are shown at the left. In the upper 

 half of the longitudinal radial section there is part of a medullary 

 ray. The lower two rows of cells of this ray are called ray tracheids. 

 They are more or less lignified, and have small, bordered pits; while 

 the rows of cells above have cellulose walls and large, simple pits. 

 The pits of the spring wood are much larger than those of the later, 

 thick-walled cells. 



Medullary rays and ray tracheids. — A striking feature of fig. 248 

 is the medullary ray, as shown in the longitudinal radial section. 

 The upper border of the ray is not shown, but it would be similar to 

 the lower. The upper three rows of cells have large, simple pits, 

 while the two rows below, the ray tracheids, have small, bordered 



