1915] FARR—INFLORESCENCES OF XANTHIUM 145 
of dicliny and of the peculiar fertile inflorescence may prove appli- 
cable to other genera of the Ambrosiaceae. 
Summary and conclusions 
1. The pistillate and staminate heads of Xanthium commune 
may be contrasted with respect to the following characters: posi- 
tion, attachment, subtending structures, number of involucral 
bracts, number of vascular bundles in the peduncle, number of 
flowers, and form of receptacle. 
2. The pistillate and staminate flowers differ in degree of devel- 
opment of pistil, corolla, and floral bract. The stamens completely 
abort in the pistillate flower. 
3. The vascular system in re peduncle of the staminate head 
has doubtless undergone reduction in the number of bundles. 
4. The number of stamens per staminate flower is probably 
now undergoing reduction. 
5. The anthers occasionally fuse, indicating relationship to the 
typical Compositae. 
6. The bur is a modified capitulum, differing from the typical 
head of Compositae chiefly in the two depressions in the receptacle. 
These pits originate through a temporary arrest of development, 
which may possibly be attributed to contact with the tips of the 
recurved involucral bracts. This recurving of the bracts may be 
the result of limited space due to the subtending structures. 
7- The spines of the bur are probably modified floral bracts. 
8. The beaks seem to be modified portions of the receptacle. 
9. The terminal heads became staminate, because the vascular 
supply was inadequate to compensate for the excessive tran- 
spiration, and hence the pistils have aborted. 
10. The axillary heads became pistillate by the abortion of 
stamens, owing to the pressure and crowding incident to the 
formation of the flowers in depressions. 
11. Many of the characters in which the pistillate and staminate 
flowers of Xanthium differ have been causative factors in the origin 
and development of dicliny in this form. 
State Universiry or Iowa 
Iowa City 
