BUNTON: TOWNSENDIA EXSCAPA. 



187 



the epidermal cells of Townsendia and a few mesophytes and 

 xerophytes : 



In the following table is given the average thickness of the 

 layer of cork, the average thickness of the walls of the cork 

 cells, and the average length and width of the cell cavity in 

 Townsendia: 



Thickness of cork 4-7 cells. 



Average thickness of wall 0.004 mm. 



Average length of cavity 0.14 mm. 



Average width of cavity 0.087 mm. 



In figure 5 is shown a small portion of cork tissue in cross 

 section. 



When compared with the mesophytes and hydrophytes 

 above mentioned, it is seen that Toivnsendia has a well devel- 

 oped protective system, one of the primary requisites of a 

 plant growing in a dry, windy habitat. 



The Strengthening Tissues. — The strengthening tissues 

 found in Toivnsendia consist exclusively of collenchyma and 

 short sclerenchyma cells. From figures 6 and 7, collenchyma 

 cells as seen in cross and longitudinal sections, it may be seen 

 that the cells of the collenchyma tissue have remarkably thick 

 walls. The walls of the short sclerenchyma cells also are much 

 thickened, as shown in figures 8 and 9. No true bast fibers are 

 found; the tissue that in cross section appeared to be bast 

 when examined longitudinally proved not to be bast, but a 

 tissue of short sclerenchyma cells. Wood fibers are also 



