192 KANSAS UNIVERSITY SCIENCE BULLETIN. 



total water-conducting area there 0.00004 sq. mm. The cross- 

 section area of the midrib was 0.01207 sq. mm., 32 per cent of 

 which was devoted to conduction of water. In figure 21b is 

 shown the midrib as seen in cross section. In the cross sec- 

 tion of each of the lateral veins were found twenty-eight 

 tracheids, the average diameter of which was 0.0059 mm., 

 making the total water-conducting area in each vein 0.00073 

 sq. mm. The total cross-section area of each vein was 0.003 

 sq. mm., 24 per cent of which was devoted to the conduction of 

 water. The average distance between the ultimate branches 

 of the veins — a and b, figure 22 — is 0.031 mm. 



In the leaf, the phloem consists only of cambiform cells and 

 undivided mother cells of the sieve tubes ; the xylem is made up 

 of wood parenchyma cells and spiral and scalariform tracheids. 

 The average diameter of the cavity of the tracheids is 0.006 

 mm. The walls of the parenchyma cells, however, remain 

 quite thin. In figure 23 are shown the phloem cells as seen in 

 longitudinal section of the midrib : o, cambiform cell ; b, undi- 

 vided mother cell of the sieve tubes. The average length of 

 these cells is 0.14 mm., the average width 0.01 mm., the aver- 

 age thickness of the cell wall 0.0006 mm. 



The Photosiinthetic Tismie^. — On both sides of the leaf are 

 found palisade cells ; the sportgy parenchyma cells are not very 

 prominent and toM^ards the center of the leaf, in some places 

 in the leaf not really being differentiated from the palisade 

 cells. The explanation of the palisade cells on both sides of 

 Townscndia may be that out on the open prairie the reflected 

 light from below is sufficiently intense to stimulate the forma- 

 tion of palisade tissue on the under side of the leaf. 



On each side of the leaf there are from two to three layers 

 of palisade cells, the average length of which is 0.14 mm., the 

 average width 0.05 mm. The average number of chloroplasts 

 in a single cell is twenty-nine and the average diameter of a 

 chloroplast is 0.0008 mm. In figure 25 are shown the palisade 

 cells as they appear in surface view; in figure 26 spongy pa- 

 renchyma cells as they appear in surface view. 



In the following table is shown the number of chloroplast^ 



