198 KANSAS UNIVERSITY SCIENCE BULLETIN. 



The bast fibers are 0.11 mm. long and 0.016 mm. wide, the 

 average thickness of the cell wall being 0.006 mm. As may be 

 seen in figure 44, the ends of the fibers overlap but little. In 

 figure 45 are shown a few bast fibers as seen in cross section. 



The most striking characteristic of the strengthening tissues 

 of the stem is the presence of large zones of thick-walled wood 

 fibers, which constitute 26 per cent of the area of the stem and 

 78 per cent of the area of the xylem. These wood fibers, as 

 shown in figure 46, occur among spiral tracheal tissues. They 

 average 0.11 mm. in length and 0.01 mm. in width. The walls 

 are remarkably thick — 0.006 mm. ; the cell cavity is exceed- 

 ingly small, in some places being almost entirely obliterated. 

 Figure 47 represents a wood fiber as seen in longitudinal sec- 

 tion. 



The strengthening system in the stem of Lesquerella is more 

 extensively developed than that in the stem of Toivnsendia, the 

 only strengthening tissues found in the stem of Toivnsendia 

 being collenchyma and short sclerenchyma cells, bast fibers and 

 wood fibers being absent. This deficiency in Townsendia, how- 

 ever, is compensated by remarkably thick cell walls, not only 

 of the strengthening tissues but of nearly all the tissues. The 

 strengthening tissues in the stem of Lesquerella are so well de- 

 veloped that there is no need of the other tissues being modi- 

 fied, as was the case in the stem of Toivnsendia. 



The Tracheal Tissues. — The tracheal system in the stem of 

 Lesquerella consists of spiral and scalariform tracheal tubes 

 and spiral and scalariform tracheids alone. There was found 

 devoted to the conduction of water 2 per cent of the stem cross 

 section, where the area was 7 sq. mm. ; 4 per cent of the xylem, 

 where the cross section area was 3.46 sq. mm., was devoted to 

 this purpose. 



In the following table is given the average thickness of the 

 cell wall, and the average tangential and radial dimensions of 

 the cell cavities of the various tracheal elements found in the 

 stem of Lesquerella : 



Average thickness Radial diametev TanRential di- 

 of cell wall. of cavities. ameter of cavities. 



Largest tracheal tubes 0.002 mm. 0.016 mm 



Smallest tracheal tubes 0.003 0.002 



Tracheids . 002 0.016 . 04 mm. 



As shown in figure 46 — a small portion of the xylem as seen 

 in cross section — the tracheal elements occur among alter- 

 nating, definite zones of wood fibers, A, and wood parenchyma 



