THE ORIGIN OF THE INFLORESCENCES OF XANTHIUM 
CLIFFORD H. FARR 
(WITH PLATE X) 
The bur of Xanthium, together with the two inclosed seeds, has 
been the subject of considerable investigation for many years. 
ARTHUR (1) in 1895 confirmed the popular notion that the germi- 
nation of one of the seeds is delayed approximately a year beyond 
the other, and also that it is the lower and better developed 
seed which germinates first. CROCKER’s experiments (5) in 1906 
indicated that this phenomenon is due to a difference in the permea- 
bility of the seed coats to oxygen. In 1911 SHULL (14) demon- 
strated further that the embryos differ in the amount of oxygen 
required for germination; and the same writer (15) has published 
an article more recently on the nature of the semipermeability of 
the seed coats. In addition to these physiological studies, the 
structure of the seed coats has been investigated by HANAUSEK (10). 
The morphology of the pistillate inflorescencé, or bur, has been 
much discussed and variously interpreted. The oldest and most 
generally accepted view is that which considers it a fusion of 
involucral bracts. This was supported by Warminc (16) and 
Rostowzew (13), and accepted by ArTHuR, Gray, and BRITTON. 
As early as 1838 Brassar (3) dissented from this interpretation 
and referred to the bur as a fusion of many floral bracts. Later 
BAILLON (2) modified Brassat’s idea by assuming a union of but 
two floral bracts, coalesced along their margins; and GorBEL in his 
most recent paper (g) accepts this explanation. However, only 
three years before, the last named writer (8) presented another 
possibility, namely, an intercalary growth about the base of the 
flowers, enveloping them and carrying their floral bracts upward to 
form the beaks. 
There is also a marked difference of opinion as to the mor- 
phology of the spines which are so prominent on the mature bur. 
Cxos (4) suggested that these are new structures (‘‘Emergenzen”’), 
Botanical Gazette, vol. 59] _ [136 
