1915] FARR—INFLORESCENCES OF XANTHIUM 141 
more advanced than the upper. This difference persists even after 
fertilization, and in every case the lower seed bears the larger 
embryo. It is evident that the lower flower is in closer connection 
with the vascular supply, and possibly this circumstance, together 
with its priority in development, may have made a difference in 
the nutrition and water relations of the two flowers. These factors 
doubtless condition to a large extent the structure and composition 
of the seed coats and embryo, which in turn have been shown to 
influence germination. It thus appears that a difference in the 
rate of growth of the two sides of the young head and consequently 
the vertical displacement of one of the seeds results ultimately in 
a difference in the periods of delayed germination. 
The sequence of development is the same for both flowers i in the 
bur. The abortive corolla forms at first a complete ring (fig. 10), 
“but before maturity it disappears entirely on the outer side 
(fig. 11). The two carpels, though appearing later, grow more 
rapidly than the corolla (fig. 10), and produce the bifid stigma which 
projects through the beaks at the time of pollination. One instance 
was noted in which two collar-like structures were present, the 
inner reaching but half-way around the base of the style. The 
outer, being a complete ring, is unquestionably the normal rudi- 
mentary corolla, and the inner should doubtless be considered the 
vestige of an abortive whorl of stamens, such as regularly appears 
in Iva (7). 
A small rudiment of a floral bract is usually noticeable on the 
outer side of the base of the pistillate flower (fig. 12). It will be 
recalled that these flowers arise in deep cavities, the walls of which 
crowd closely about them on all sides. There is scarcely any space 
for the development of a floral bract therefore, and probably this 
crowding accounts for its reduction to the present dwarfed condi- 
tion. Its presence, nevertheless, even though much reduced, 
precludes the possibility of considering the beaks to be the floral 
bracts of the subtended flowers. 
Discussion 
If the bur of Xanthium in its individual development follows 
at all closely,the course of its evolution, there seems no doubt of 
its being a modified capitulum. The involucral bracts arise in the 
