dalbey: some species of xanthium. 61 



The cell walls vary in thickness from .0015 mm. to .002 mm., 

 and are cellulose, except those near the wood, which have 

 lignified walls. In each species the average diameter of the 

 pith in number of cells varies from 30 to 35. 



Clusters of calcium oxalate crystals, with an average di- 

 ameter of .015 mm., are regularly distributed throughout the 

 pith (figs. 32, N; 33, 0). The average number per sq. mm. 

 seen in cross section varies in the three species : in X. pennsyl- 

 vanicum 1000, in X. americanum 234.5, and in X. globosum 

 493.8. 



The pith cells of each species contain a large amount of 

 stored-up material in the form of glucosides. Sections boiled 

 in Fehling's solution did not show a reaction for sugar, but 

 when sealed on slides in Fehling's solution, and kept at a rather 

 high temperature for about forty-five minutes, crystals of 

 copper oxide appeared, indicating the hydrolysis of glucosides. 



The wood increases in size by the interpolation of new 

 bundles, the bundles of one section of a growing stem showing 

 a large variation in size (fig. 27). In X. americanum and 

 X. globosum the ray cells between the bases of the bundles are 

 elongated parallel to the circumference (fig. 41, N) . X. penn- 

 sylvanicum does not show this characteristic. 



In cross section the vessels are seen in radial rows (figs. 

 38, 39 and 40) , with the rows of vessels in the large bundles at 

 the angles of the stem (fig. 28) separated by a greater amount 

 of wood parenchyma than in other bundles of the same stem 

 (fig. 27). The vessels varied in diameter in a single bundle 

 from .010 to .060 in X. pennsylvanicum, .017 to .052 in X. 

 americanum, and .015 to .038 in X. globosum. The species 

 vary in the per cent of stem area made up of vessels. In X. 

 pennsylvanicum and X. globosum the water-conducting tissue 

 composes 2 per cent of the stem area, and in X. americanum 

 3 per cent. 



Wood parenchyma is scanty in amount (figs. 38, 39, 40), 

 and the stem is made strong by a large amount of lignified 

 wood fibers (fig, 42, P) . In X. pennsylvanicum 90.5 per cent 

 of the xylem is lignified, in X. americanum 85.1 per cent, and in 

 X. globosum 86.7 per cent. 



The wood fibers average in diameter .0113 mm. in X. penn- 

 sylvanicum, and .0110 mm. in X. americanum and in X. globo- 

 sum. 



3 — Univ. Sei. Bull., Vol. IX, No. 5. 



