1915] FARR—INFLORESCENCES OF XANTHIUM 139 
in the same manner as in Iva (7). The fused cutinized layer is, 
however, not quite as thick as in the latter form. The similarity 
between Xanthium and Iva further extends to the ringlike enlarge- 
ment at the base of the abortive style. These facts only serve to 
emphasize the kinship of these two genera with the main body of 
the Compositae. 
RostowzEw (13) stated that the stigma of the abortive style 
probably does not represent two carpels. In Iva, though this 
abortive stigma is not bifid at maturity, the appearance of a notch 
at the apex during development indicates derivation from the 
typical bifid form. Since no such notch appears in Xanthium, it 
seems probable that this genus has gone one step farther and oblit- 
erated even this last clue to the evolution of this structure. The 
close relationship of this form of Jva, however, strongly suggests 
that this rudimentary style arose from the usual bifid form. 
The pistillate inflorescence 
The fertile flower is axillary, being subtended (figs. 4-6) by a 
leaf or another pistillate head. Many instances of aborting leaves 
in such relation explain the frequent absence of subtending struc- 
tures at maturity. The nearly sessile capitulum is attached to the 
floral axis by a large base which is over twice the diameter of the 
peduncle of the staminate head and contains 24 bundles (fig. 5). 
This vascular supply, in contrast to that of the staminate head, 
seems remarkably extensive when it is recalled that the pistillate 
head bears only two flowers, while the staminate has 150 or more. 
The involucral bracts, 9-15 in number, appear first and arch 
over the young head, protecting it very effectively. The proximal 
ends of the adaxial bracts are closely crowded between the recep- 
tacle and the stem, and those on the abaxial side between the 
receptacle and the subtending structure. Meanwhile the tips of 
these recurved bracts (fig. 6) come in touch with the apex of the 
growing receptacle near its center. It seems that this contact 
temporarily arrests development in that region of the receptacle, 
for it immediately becomes flattened. Soon continued growth of 
the margin of the receptacle results in the formation of an apical 
depression (fig. 7). It may be, of course, that this sequence of 
