Time of Floral Differentiation in Ulmus americana, U. pumila, and U. carpinifolia

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Abstract
1. Buds from flowering trees and young non-flowering trees of Ulmus americana L., U. pumila L., and U. carpinifolia Gleditsch were collected through two growing seasons. The collections were made at intervals from May 9, 1955, to February 24, 1956, and from June 5, 1956, to July 25, 1956. 2. Stained microtome sections of these buds were examined to learn the time of floral differentiation. No evidence of floral initials could be found in young non-flowering trees. In buds from flowering trees the earliest floral differentiation observed was in a bud collected from U. carpinifolia on June 14, 1955; the latest, in a collection from U. americana made on August 10, 1955. Although the spring of 1955 was an unusually early one for Columbus, Ohio, floral differentiation did not occur earlier than in the succeeding year. 3. The three species were uniform in development in the autumn and winter. In October, differentiation of the megaspore and microspore mother cells was observed. The winter was passed in this stage. In February, the microspore mother cells were still undivided, and no enlargement of the megaspore mother cell was apparent. First evidence of differentiation of the inner integument was found in the February collections.

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