Effect of Length of Induction Period on Floral Development of Xanthium pennsylvanicum

1. Histological examinations were made of the developing carpellate and staminate inflorescences of Xanthium pennsylvanicum. The plants were exposed after the twenty-fifth day of vegetative growth to (a) continuous photoinductive cycles, (b) one photoinductive cycle, (c) four photoinductive cycles, (d) eight photoinductive cycles. The study included those inflorescences which developed from the terminal bud and the buds in the axils of the five uppermost leaves. 2. Development of both carpellate and staminate inflorescences in the plants exposed continuously to photoinductive cycles was more rapid than that in any other group. All flower parts were present by the thirteenth day, and at the end of the month the seeds were almost mature. 3. Development of inflorescences in the plants which received only one photoinductive cycle was much slower than that of plants which received continuous photoinductive treatment. The flower parts of the former did not appear in all the flowers of the terminal staminate inflorescence until the sixty-fourth day after induction, rather than by the thirteenth day as in the latter. The former also differed from the latter in that there was a greater difference in the size and stage of development between the outer and inner flowers of the inflorescence. 4. The terminal staminate inflorescence of the plants receiving one photoinductive cycle, as well as of the plants exposed continuously to photoinductive treatment, contained one or two axes, each appearing in the axil of an involucral bract of the terminal inflorescence. Each axis of the former was composed of three staminate inflorescences, rather than of a single one as in the latter. The small staminate inflorescences of the former were much more elongated than those of the latter. 5. In the axil of the uppermost leaf a lateral axis consisting of three staminate inflorescences appeared in some of the plants exposed to one photoinductive cycle, while in those exposed continuously to photoinductive treatment an individual carpellate inflorescence or an axis consisting of three usually occupied this position. The staminate inflorescences of the former were elongated rather than round. 6. In the plants exposed to one photoinductive cycle there developed approximately ten staminate and six carpellate inflorescences, while in the plants exposed continuously to photoinductive cycles there was an average of about four staminate and twelve or thirteen carpellate inflorescences per plant. Thus, increasing the number of induction treatments seemed to stimulate the production of carpellate more than of staminate inflorescences. 7. The plants which received four photoinductive cycles exhibited, in the axils of involucral bracts of the terminal staminate inflorescence, one or two staminate inflorescences three times as large as those occupying this position in plants exposed continuously to photoinductive treatment. 8. The development of the inflorescences on those plants which received four or eight photoinductive cycles resembled more closely the development in plants continuously exposed to such treatment than that in plants exposed to one photoinductive cycle. The rate of development was intermediate between the two.