202 KANSAS UNIVERSITY SCIENCE BULLETIN. 



In figure 60 — a diagrammatic drawing of the cross section 

 of the root traced as seen when projected upon a screen — are 

 shown the different zones of tissues of which the root con- 

 sists and their relation to one another: A, cork; B, collen- 

 chyma ; C, stone cells ; D, thin- walled parenchyma ; E, phloem ; 

 F, wood fibers; G, wood parenchyma; H, tracheal elements; 

 /, medullary ray. 



The Tegumentary Tissues. — The tegumentary tissues con- 

 sist of an epidermis with trichomes. All the walls of the epi- 

 dermis are cellulose except the outer tangential wall, which has 

 a thin film of cutin on the outer portion. The outer and inner 

 tangential wall each measures in thickness 0.006 mm., and the 

 radial wall 0.003 mm. The average radial diameter of the cell 

 cavity is 0.02 mm., the average tangential diameter 0.016 mm. 

 On both sides of the leaf the epidermis is of the same charac- 

 ter. Figure 61 represents a small portion of the epidermis: 

 a, cutinized portion; b, cellulose portion. The trichomes are 

 thickly distributed over both sides of the leaf, and must be 

 quite effective in giving protection. The discussion of these 

 trichomes will be taken up in connection with the water- 

 storage system. 



The Tracheal Tissues. — For the study of venation, a leaf was 

 used which had been bleached by standing four or five hours 

 in dilute hydrochloric acid and then two or three days in 

 chloral hydrate. As shown in figure 62, there could be seen 

 in this leaf one large central vein, A, running longitudinally 

 throughout the length of the leaf and sending off near the 

 center of the leaf two large lateral branches, B, and below the 

 center the large branch, G. On either side of the midrib there 

 are two prominent longitudinal veins, C. These primary veins 

 send out other veins which branch in every direction and end 

 abruptly, as shown at D. The average distance between the 

 ultimate branches of the veins, E and F, is 0.16 mm., there 

 being included within this area, on the average, twenty-five 

 mesophyll cells. The phloem consists only of undivided mother 

 cells of the sieve tubes, whose average length is 0.16 mm. and 

 width 0.006 mm. The xylem consists exclusively of spiral 

 tracheary tracheids. The average diameter of the tracheids 

 in the midrib is 0.006 mm., and the average diameter of those 

 in the smaller veins is 0.003 mm. In the midrib, where the 

 cross-section area is 0.015 sq. mm., 20 per cent is devoted to 



