THE ROOT 147 



external activity of the cambium. The primary elements of the 

 phloem constitute a distinct band of collapsed cells on the upper 

 and lower sides of the cylinder, and this is usually very conspicuous 

 in sections of the mature root, provided the organ has not become 

 too old. Within the zones of radially disposed secondary phloem 

 lies the secondary xylem, which has its cells similarly arranged in 

 series. The two masses of secondary wood, facing the correspond- 

 ing aggregations of secondary phloem, are joined internally with 

 the primary xylem, and they are readily distinguishable from this 

 by the presence of rays and the regular disposition of the tracheary 

 elements. The primary xylem is now complete and no longer shows 

 the hiatus or interruption in the center which is found in the 

 younger root. The two resin canals which lie opposite the groups 

 of small tracheids constituting the protoxylem have not collapsed. 

 The primary wood in the root, in contrast to the primary phloem, 

 does not break down, but persists in maturity. Meanwhile in the 

 exterior or cortical region of the root an important change has 

 taken place in consequence of the formation of a layer of impervious 

 secondary tissue known as periderm. This zone, by reason of its 

 imperviousness, prevents the conveyance of water and nourishment 

 to the outer portion of the root, and the latter consequently dies 

 and is sooner or later exfoliated as a shriveled mass of collapsed 

 cells, as indicated in the diagram (Fig. 103). The result of the loss 

 of the cortex and piliferous layer in the older roots of the conifers 

 is to bring the fibrovascular tissues in immediate relation to the 

 soil. In this stage, however, they no longer serve the function 

 of absorption, but only that of conduction, since -the taking up of 

 water and nutritive substances in solution is an activity of the 

 younger root when it is still provided with root hairs. 



The subject of the structure and development of the root is so 

 important from the evolutionary standpoint that it is advisable 

 to supplement the description furnished in the two preceding some- 

 what diagrammatic drawings (Figs. 102 and 103) of the organization 

 of the root with actual photographic reproductions of the struc- 

 tures present. In order that the general validity of the principles 

 involved may be made apparent, a different genus of the conifers 

 has been purposely chosen. The diagrams (Figs. 102 and 103) refer 



