64 KANSAS UNIVERSITY SCIENCE BULLETIN. 



The phloem varies in thickness in the three species. In X. 

 pennsylvanicum it is .150 mm., in X. americanum .202 mm., 

 and in X. globosum .195 mm. in thickness. 



In X. pennsylvanicum and X. globosum the phloem at two 

 opposite sides of the root is thicker than at other parts of the 

 root (figs. 60 and 61), and extends into the xylem. X. ameri- 

 canum does not have this characteristic. The phloem in each 

 species has irregular areas in which the cell w^alls are lignified. 



SUMMARY. 



1. The three types of Xanthium described present some 

 striking external characteristics, while in their anatomy there 

 are some definite but minor differences w^hich might prove of 

 uncertain value in classification. 



2. In each species three large veins enter the leaf, the mid- 

 rib and other large veins containing a very large amount of 

 thin-w r alled parenchyma cells in which resin ducts occur. 



3. The leaf of Xanthium pennsylvanicum has a little more 

 palisade tissue than the leaf of Xanthium americanum and 

 Xanthium globosum. 



4. The stomata are more numerous in X. americanum than 

 in X. pennsylvanicum or X. globosum,. 



5. Numerous trichomes of two different forms occur on 

 both surfaces of the leaf. The most conspicuous is relatively 

 large, awl-shaped, and composed of three cells ; the other form 

 is linear. These always occur in connection with the water- 

 storage cells above and below the veins. 



6. A cuticle layer covers the entire outer wall of the hairs 

 in each species. 



7. The young stems are slightly five-angled in outline, with 

 X. globosum containing a little more wood than the other two 

 species described. 



8. X. americanum and X. globosum have the ray cells be- 

 tween the bases of the bundles elongated tangentially. X. 

 pennsylvanicum does not show this characteristic. 



9. X. americanum has 3 per cent of the stem area given to 

 vessels; X. pennsylvanicum and X. globosum have 2 per cent. 



10. The phloem is not differentiated into sieve tubes and 

 companion cells. 



11. In each species resin ducts occur in the parenchyma of 

 the pericycle. 



