THE ROOT 151 



the topography of the root in any living representative of the fern 

 alliance or of the Pteridophyta in general. 



The structures of the root in the dicotyledonous angiosperms 

 are appropriately considered at this stage. Since the early condi- 

 tion of development of the root has been already discussed and 

 figured in detail for the conifers, it will not be necessary to enlarge 

 upon this phase in the present connection, for the underlying 

 principles involved are the same in both conifers and dicotyledons. 

 It will be convenient to begin with a woody or arboreal root. 



FIG. 107. Transverse section of root in Alnus incana. Explanation in the 

 text. 



Fig. 107 illustrates the organization of an older root in Alnus incana. 

 Toward the right appears a layer of periderm bounding the outside 

 of the organ. Within lies the cortex, which terminates at a layer 

 of thick-walled cells, the pericycle. Within the pericyclic layer, 

 marking the outside of the fibrovascular cylinder, lies the phloem, 

 not depicted in detail. Then comes the cambium, followed in- 

 ternally by the secondary wood, in which conspicuous rays are 

 present. Toward the extreme left of the illustration lies the five- 

 angled star indicating the topography of the primary xylem. The 

 stellate aggregation of elements shows at each angle of its five 

 points groups of cells of smaller size, the clusters of protoxylem 

 consisting of spiral tracheids. The mass of metaxylem into which 



