146 THE ANATOMY OF WOODY PLANTS 



the oval area between the cambial bands lies the primary xylem. 

 This consists of smaller elements at the two external angles, the 

 protoxylem strands. The development of the primary wood is not 

 complete; hence there is a considerable interval in the center 

 which will be later transformed into tracheids of the primary 

 metaxylem. In the first-formed elements of the primary wood the 

 bordered pits of the tracheids can be plainly seen. Subtending each 

 cluster of protoxylem is a resin canal, a common feature of the 

 organization of the root in Picea and allied genera of the Abietineae. 

 It is manifest from the general situation represented in the median 

 region or central cylinder of the root that the primary structures 

 are the only ones conspicuously in evidence, and of these the wood 

 is still incomplete. Further, there is a clear indication of the 

 presence of cambial layers, which will a little later give rise to sec- 

 ondary elements of both wood and bast. Surrounding the central 

 cylinder or stele of the root is the cortex, which is limited internally 

 by the endodermis and externally abuts on the root-hair-producing 

 stratum of the root: this is ordinarily called the piliferous layer. 

 As the section is taken very near the apex of the root, a few rather 

 poorly developed root hairs are seen. It has often been wrongly 

 asserted that there are no root hairs present in coniferous roots. 

 While this condition may be found in the case of those fungus- 

 inhabited radical organs known as mycorrhizae, carefully excavated 

 normal coniferous roots nearly always show a band of root hairs 

 in proximity to the root cap or pileorhiza. It will be made clear in 

 what follows that both root hairs and cortex are of short duration 

 in coniferous roots. 



In the older root marked differences present themselves both 

 in the region of the central cylinder and in the cortex. Taking first 

 the fibrovascular tissues, we find that the pericycle is as well marked 

 as in the younger condition of the organ, but is now characterized 

 by the formation in its outer region of a zone of regularly arranged 

 cells known as the periderm. This layer has an important influ- 

 ence, as will be indicated later, on the tissues constituting the outer 

 region of the root. It is clear that the primary phloem has more 

 centrally been superseded by a mass of tissue of very regular radial 

 arrangement, the secondary phloem, derived as a result of the 



