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

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Abstract
The plants were exposed to (a) continuous photoinductive cycles, (b) 1, (c) 4, (d) 8 photoinductive cycles. Histo-logical examinations were made of the developing carpellate and staminate inflorescences. The study included those inflorescences which developed from the terminal bud and the buds in the axils of the 5 uppermost leaves. Development in the plants of group (a) was much more rapid than that in any other group. The plants of group (b) differed from (a) in that there was a greater difference in the size and stage of development between the outer and inner flowers of the inflorescences. In plants of both (a) and (b) the terminal staminate inflorescence contained axes in the axils of involucral bracts. Each axis of (b) was composed of 3 staminate inflorescences, rather than of 1 as in (a). The small staminate inflorescences in (b) were more elongated than those of (a). In some plants of (b) a lateral axis consisting of 3 elongated staminate inflorescences appeared in the axil of the uppermost leaf. In (a) either an individual, or 3, carpellate inflorescences occupied this position. In (b) there developed approx. 10 staminate and 6 carpellate inflorescences, while in (a) there was an average of about 4 staminate and 12-13 carpellate inflorescences per plant. Thus, increasing the number of induction treatments seemed to stimulate the production of carpellate more than of staminate inflorescences. The development of the inflorescences of (c) and (d) resembled more closely that of (a) than that of (b). The rate of development was intermediate between the two.

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