Anatomy and Histology. 91 



tangential increase in size results in (i) extension of the radial dimensions 

 of the so-placed walls. This extension is confined to that part of the wall 

 between its outer limit and the band of Caspary, leaving this band in the 

 same position as before (plate 22, figs. 6, 8). With this fact in mind the 

 endodermis may be identified for a long time, indeed frequently till it 

 is well-nigh expelled by secondary thickening. (2) Tangential growth is 

 accompanied by cell-divisions in the radial direction, Casparian bands 

 being formed in the new walls (plate 22, fig. 6). In particular positions, 

 namely, opposite the leptome bundles, the earliest i resin-canals appear. 2 

 These do not belong to the primary structure of the root, but arise second- 

 arily in the endodermis (plate 22, figs. 1 to 5). Their mode of development 

 is as follows: 3 or 4 adjacent cells divide by periclinal walls, thus bringing 

 it about that two or three places occur where 4 cells lie with their angles 

 adjacent. Here the walls split apart, making a simple, prismatic, inter- 

 cellular space without demonstrably different contents ; the adjacent cells 

 divide radially, so that each canal has now 4 cells contingent upon and 

 peculiar to it. Later, further divisions, roughly parallel to the early canal- 

 walls, result in the canal consisting structurally of two layers of cells 

 constituting a prismatic tube. It is seen that the endodermal canals, for 

 so they will be called, are arranged in two groups, of usually two or often 

 three or occasionally even four canals each, each group being placed oppo- 

 site a primary leptome bundle. This relation was first described by von 

 Sachs for Helianthus. The structure of these canals does not change, 

 though there ensues some displacement of the cells in roots which have 

 thickened abnormally without losing the outer primary tissues. The figures 

 displaying the cell lineage will make this clear (plate 22, figs. 1 to 5). 



The growth of the endodermis may be followed till it is thrown off 

 by the formation of cork within it. During its history it enlarges from 

 a cylinder, of 0.1 mm. inside diameter, of 18 to 20 cells, to one of 3 mm. 

 diameter, composed of hundreds of cells, or even to larger dimensions, 

 before being finally cut out. Throughout the greater portion only radial 

 divisions occur, though the cells increase in radial depth. In the region 

 of the canals, however, the endodermal cells divide in a general periclinal 

 direction, giving rise to two or even more irregular series of cells. 



In the ultimate condition of the endodermis and of the secondary 

 cortex the walls of the cells are reticulately thickened (plate 22, fig. 15), 

 so that in a root 2 mm. in diameter, of a field seedling, the endodermis 

 may be followed all the way around with great ease, provided that the 

 rubber has been previously extracted. The reticulations are the result 

 of the oval form of the broad, shallow pores, which are somewhat crowded. 

 They are more strongly developed in plants grown under normal condi- 

 tions, as appears from the fact that in an irrigated seedling, with a root 



1 Ross (1908, p. 25), in stating that there are only a few canals in the primary 

 cortex of the root, does not make it clear that he refers to canals of endodermal 

 origin. 



2 Exceptions occasionally occur in which the canals in one half-circle of the 

 hypocotyl do not approach on entering the root, and conversely, cases occur in 

 which the grouping of the canals occurs in the hypocotyl, on one side of it. In 

 other words, the root-structure is taken on at a higher level on one side than on 

 the other. 



