BUNTON: TOWNSENDIA EXSCAPA. 191 



by Townsendia as fast as it is made. No statement, however, 

 can be made of the storage of food in the plant later in the 

 season. 



The Root. 



Tracheal Tissues. — As in the stem, the conduction of water 

 in the root is provided for by spiral and scalariform tracheids. 

 By computing the number of tracheids in a certain portion of 

 a cross section, and multiplying by the number of such por- 

 tions, there were found to be 1271 tracheids in the cross sec- 

 tion of the root. In figure 20 may be seen how these tracheids 

 are distributed throughout the section. The average diame- 

 ter of these tracheids is 0.018 mm. The total water-conduct- 

 ing area in a root, where the cross section area is 7.496 sq. 

 mm., is 0.325 sq. mm., or 4.2 per cent of the whole root area. 



In structure, the other tissues of the root do not differ es- 

 sentially from those of the stem ; hence the discussion of them 

 as found in the stem will suffice for the root. 



The Leaf. 



The Tegumentary Tissues. — The outer wall of the epidermis 

 of the leaf is not wholly cutinized ; the inner portion is of cel- 

 lulose and is 0.0108 mm. thick, and the outer portion of the wall 

 is cutinized and is 0.0072 mm. thick. The inner tangential 

 wall is of cellulose and is 0.0036 mm. thick. The radial walls 

 are likewise of cellulose and 0.0036 mm. thick. The average 

 tangential length of the cells of the epidermis is 0.0396 mm. ; 

 the average radial length, 0.025 mm. No difference was found 

 between the upper and lower epidermis, hence the figures given 

 above will suffice for both. In figure 24 is shown a portion of 

 the cutinized epidermis as seen in cross section. 



Tracheal Tissues. — After a leaf boiled in alcohol had re- 

 mained in a mixture of chloral hydrate and hydrogen peroxide 

 ■for several days, it had become so well bleached that its vena- 

 tion could be seen easily with the naked eye. There appeared 

 three prominent longitudinal veins, one central and two lat- 

 eral, from which extended other veins branching in every 

 direction and terminating in water-storage tracheids, as 

 shown in figure 21a. In the cross section of the midrib 

 through the center of the leaf were found ninety-five tracheids, 

 the average diameter of whiqh was 0.0075 mm., making th^ 



