BUNTON: LESQUERELLA SPATHULATA. 199 



cells, B, the largest ones being among the wood fibers and the 

 smallest among wood parenchyma cells. It is generally con- 

 ceded that spiral vessels are formed by the procambium ex- 

 clusively, and wood fibers by the cambium. As the tracheal 

 elements found interspersed among the wood fibers are spiral 

 and scalariform, the activity here must be due either to the 

 cambium or procambium. From the fact that these spiral 

 vessels are found somewhat remote from the pith, there fol- 

 lows a probable conclusion that the cambium, contrary to the 

 general rule, has formed spiral vessels. 



The tracheal elements found in the stem at the nodes were 

 different from those in the internodes. At the nodes the 

 tracheal tissues consist exclusively of short and peculiarly 

 shaped tracheids. In figure 48, A, are shown the scalariform 

 type of tracheids; B, the spiral type of tracheids as seen in 

 longitudinal section. 



In the internodes the tracheal elements consist exclusively of 

 spiral and pitted tracheal tubes, with the exception of a 

 few tracheids interspersed here and there. Figure 49, A, illus- 

 trates the spjral tracheal tubes found among the wood fibers, 

 as seen in longitudinal section; figure 49, B, spiral tracheal 

 tube found among the wood parenchyma cells; figure 49, C, 

 pitted tracheal tubes as seen in longitudinal section. 



Tissues for the Conductioji and Storage of Food. — The med- 

 ullary rays found in the stem of Lesquerella present an inter- 

 esting characteristic. Throughout the entire length of the 

 medullary ray are found leaf traces containing spiral tracheal 

 tubes as shown in figure 51, a. These tracheal tubes are much 

 smaller than those found in the xylem area of the stem, the 

 average diameter of the cavity being 0.007 mm. and the aver- 

 age thickness of the cell wall 0.002 mm. Figure 50, a repre- 

 sents a leaf trace. The internodes are so short that it is im- 

 possible to cut a section without having leaf traces traversing 

 the medullary rays. These leaf traces run horizontally into the 

 primary xylem, and when the cambium begins its activity it 

 builds up the medullary rays around them. The average 

 width of a medullary ray is 0.06 mm., and the average vertical 

 length of a medullary-ray cell is 0.03 mm., and its average 

 width is 0.01 mm. Figure 51, h represents a small portion of 

 the medullary ray as seen in cross section. 



In the stem, the ordinary collateral vascular bundles are 



