BUNTON: LESQUERELLA SPATHULATA. 201 



average thickness of the cell walls of the tracheal elements in 

 the root: 



Average diameter of the larger tracheal tubes 0.03 mm. 



Average diameter of the smaller tracheal tubes 0.007 



Average thickness of cell wall . 005 



As in the stem the tracheal elements of the root are inter- 

 spersed among definite, alternating layers of wood fibers and 

 wood parenchj'ma cells. Figure 55 represents a few wood 

 fibers and tracheal tubes as found in the outermost layer of 

 xylem (fig. 56a, E) as seen in cross section. Here the tracheal 

 tubes are relatively large and the wood fibers have compara- 

 tively large cavities. The next succeeding layer of xylem 

 (fig. 56a, F) consists of very small, thick-walled tracheal tubes, 

 A, and wood parenchyma cells, B, shown with greater magnifi- 

 cation in figure 57. In figure 58 are shown a few wood paren- 

 chyma cells as seen in longitudinal section. The average length 

 of these cells is 0.07 mm. and the average width 0.015 mm. 

 Figure 59 represents a few wood fibers and tracheal tubes 

 found in the next succeeding layer of xylem (fig, 56a, G) . The 

 average diameter of the tracheal tubes in this layer of xylem is 

 0.023 mm. ; here the wood fibers have remarkably thickened 

 walls and a smaller cavity than those found in the outermost 

 layer of xylem (fig. 56a, E). 



Figure 56a, a segment of the cross section of the root, shows 

 the constitution of the xylem: A, wood fibers; B, tracheal ele- 

 ments ; C, wood parenchyma. 



In figure 56& are shown the tracheal elements found in the 

 root: A and B, scalariform tracheal tubes, C, spiral tracheal 

 tube. 



Tissues for the Conduction a7id Storage of Food. — In the 

 root are found only secondary medullary rays, which are nar- 

 rower than those found in the stem. In these medullary rays is 

 found no tracheal system as in those of the stem. As in the 

 stem, there seems to have been made very little provision for 

 the passage of food longitudinally. Yet, however, a larger pro- 

 portion (25 per cent) of the vascular bundle of the root is de- 

 voted to phloem than in the stem. The phloem consists ex- 

 clusively of undivided mother cells of the sieve tubes. 



Starch and proteids in small quantities were found in the 

 medullary rays and phloem-parenchyma cells, but neither glu- 

 cose nor oil. In the root of Toiunsendia there was found 

 glucose, but neither starch, proteid nor oil. 



