2OO 



THE ANATOMY OF WOODY PLANTS 



rewluta. Toward the lower side of the figure is seen the phloem, 

 composed of elements which in the main are arranged in regular 

 radial rows indicating an origin from cambial activity. The upper 

 region of the bundle is composed of xylem, consisting of very large 

 elements which become progressively smaller and fewer in number 

 in the downward direction. The lower point of this aggregation of 

 cells is the protoxylem. This region is surrounded inferiorly by 

 a few rows of parenchyma which give place in turn to more 



FIG. 147. Transverse section of leaf of Cycas revoluta. Explanation in the text 



thick-walled elements of the xylem, in contact with the regular 

 radial files of the cambium. The lower region of the xylem is the 

 so-called centrifugal xylem; the upper portion broadening from the 

 narrow protoxylem is the centripetal or cryptogamic xylem. The 

 first-mentioned group of elements of the xylem are pitted in their 

 character, as will be made clear in a subsequent longitudinal illus- 

 tration. The latter, the so-called cryptogamic wood, is of impor- 

 tance from the evolutionary standpoint because it indicates at once 

 a clear relationship on the part of the Cycadales to the vascular 

 cryptogams and to the lower and extinct gymnosperms. In the 

 next figure (Fig. 147) is represented a transverse section of one of the 



