io FIELD MUSEUM OF NATURAL HISTORY BOTANY, VOL. IV. 



achenes in a single head frequently germinate, usually not at the same 

 time however, so the seedlings will be at two stages of growth." In 

 1895, Arthur (Proc. Soc. Prom. Agricult. Science 16:70-79) remarked 

 upon the inequality in size between the two achenes in the burs of 

 Xanthium and the delayed germination of the smaller one. Arthur 

 stated (loc, tit., p. 77) that "the bur with its contents is the physiological 

 equivalent of an achenium." Later, Masterman (Ohio Nat. 1:69-70. 

 1901) published observations upon several thousand specimens which 

 showed results directly opposite to those of Arthur (thus, of 3000 burs, 

 2751 produced two plants each, at the same time). 



In 1906, Crocker (Bot. Gaz. 42:265-291), writing upon delayed 

 germination of seeds, treated of the physiological conditions of delayed 

 germination of Xanthium seeds. In 1908, Bitter, who had been inspired 

 by the early observations of Lasch (Bot. Zeitung 14:409. 1856) to 

 undertake careful cultural researches upon certain species of Xanthium, 

 was able to describe several pronounced races of X. italicum and of 

 X. spinosum. His numerous attempts at crossing X. spinosum with 

 species of the Section Euxanthium had been in vain. Thus he states 

 (Abhandl. Nat. Ver. Bremen 19": 291. 1908): "Bastarde lassen sich 

 nach meinen Erfahrungen zwischen den Euxanthien leicht erzielen, 

 wenigstens habe ich X. macrocarpum DC. und X. italicum erfolgreich 

 mit X. strumarium gekreuzt; dagegen waren meine vielfachen Versuche, 

 X. spinosum mit Euxanthien zu kreuzen, erfolglos." 



Somewhat later in the same year, Thellung (Verhandl. Bot. Verein 

 Brandenb. 50": 137-151; see also Mitteil. Bot. Mus. Univ. Zurich 58: 

 505-512. 1912) published a very comprehensive survey of the botanical 

 literature relating to X. orientale L. and X. echinatum Murr. Thellung's 

 concept of these two species was so broad that his treatment of them 

 was stretched almost into monographic proportions. His work in the 

 herbarium seems to have been far less extensive than his work in the 

 library. We are not able otherwise to explain many of his numerous 

 equations of species with each other, species which in several in- 

 stances are so strikingly unlike in fruit characters that we are entirely 

 unable to accept their reduction to synonomy by Thellung. 



In 1911, Shull (Bot. Gaz. 52:453-477) published his investigations 

 upon the oxygen minimum and the germination of Xanthium seeds. 

 Later, in 1914, the same writer (Bot. Gaz. 57 : 64-69) presented fur- 

 ther data obtained along these lines. Likewise in 1914, Dalbey (Kansas 

 Univ. Science Bull. 9 : 57-65 and pis. 16-22) enumerated various 

 anatomical characters of X. pennsylvanicum, X. americanum [ = X. 

 chinense] and X. globosum. She states that these three species "pre- 

 sent some striking external characteristics, while in their anatomy there 



