BUNTON: TOWNSENDIA EXSCAPA. 195 



acter of the stomata on either side of the leaf. It seems rather 

 surprising to find in the leaf of a xerophyte the usual number 

 of stomata found in plants and to find them at the surface of 

 the leaf instead of being sunken beneath the epidermis; for, 

 as a rule, in xerophytic plants there are fewer stomata and 

 they are often sunken beneath the epidermis. 



In figure 35 are shown the stomata as they appear in the 

 surface view of the leaf of Toivnsendia. In figure 36 is shown 

 a stoma as it appears in cross section of the leaf. 



SUMMARY. 



1. Size and Habit of Toivnsendia. — Townsendia is a low 

 plant, growing from one to four inches high; it grows on the 

 flat, open prairie as well as in the draws, and occurs not in 

 clumps but as isolated individuals. The stem is at first rela- 

 tively simple, but, as a rule, it becomes more and more 

 branched with age until the whole plant may attain a breadth 

 of ten to twelve inches. 



2. Tegumentary Tissues. — In the mature stem of Town- 

 sendia the cutinized epidermis, whose outer walls measure 

 0.05 mm., and the cork tissue several layers thick, constitute 

 the tegumentary tissues. 



In the leaf of Toivnsendia there is found an epidermis 

 whose outer wall measures 0.017 mm. in thickness. The outer 

 portion of the wall is cutinized and measures 0.0072 mm. in 

 thickness ; the inner portion is of cellulose and measures 

 0.0108 mm. in thickness. 



3. Strengthening Tissues. — In Toivnsendia the strengthen- 

 ing tissues of the stem consist of collenchyma and short 

 sclerenchyma cells. True bast fibers and wood fibers are lack- 

 ing. The remarkably thick cell walls, not only of the strength- 

 ening tissues but of nearly all the tissues, make the stem of 

 Toivnsendia strong. 



4. Tracheal Tissues. — The area devoted to conduction of 

 water in Townsendia is comparatively small, there being only 

 1.3 per cent of the stem devoted to that function. Growing as 

 it does in a location where the water supply is poor, Toivn- 

 sendia would not require very extensive water-conducting 

 tissues. 



5. Food-conducting Tissues. — Longitudinally there seems 

 to have been made very little provision for the conduction of 



