Floral Morphology of Populus deltoides and P. tremuloides. Contributions from the Hull Botanical Laboratory 639

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Abstract
Various aspects of floral morph. of P. deltoides, in which a time lag may occur between the maturation of staminate and pistillate catkins, were compared with those of P. tremuloides in which this lag does not occur. Bracts of both flowers are foliar in nature, and the vascular supply to the bract passes off from the cylinder in the pedicel as a single strand, leaving a single gap. In both flowers the disk is abundantly supplied with vascular tissue. In the pistillate flower, however, only the inner tissue of the disk is supplied with vascular bundles. Each stamen receives a single bundle; each carpel receives 1 dorsal and 2 ventral bundles. Microsporogenesis and anther development follow the usual angiosperm pattern. Stamens pass the winter in the spore-mother-cell stage. Stomata occur on the connective and filaments in P. deltoides. Nucellar primordia are conspicuous in winter pistilate buds, but the integuments do not appear until spring. In P. deltoides the no. of carpels varies from 2 to 4. The ovules are initially bitegmic, but in older ovules only the outer can be distinguished. In P. tremuloides there are only 2 carpels, and the ovules are unitegmic. The ovules are anatropous, and each has a well-developed vascular strand. In P. deltoides there are 5 or 6 archesporial cells, but usually only 1 develops. In P. tremuloides there is a single archesporial cell. It gives rise to a parietal cell, which divides several times in P. deltoides and only once in P. tremuloides, and a sporogenous cell. The latter becomes the megaspore mother cell which gives rise usually to a T-shaped, or occasionally to a linear, tetrad of megaspores. Development of the megagametophyte corresponds to the Polygonum type. During the later stages of development the megagametophyte projects through the nucellus into the micropyle. Penetration of the pollen tube is porogamous. Double fertilization occurs. Division of the primary endosperm nucleus precedes that of the zygote which forms the embryo according to the Onagrad type. Endosperm is nuclear. The cotton is composed of epidermal hairs of the placentae, not 6i the seeds, and pieces of the placenta are shed with the seeds at maturity. Pollen germination tests in P. deltoides indicate that pollen grains readily germinate when they are fresh. No germination was obtained from any pollen which had been stored, even when stored for only 3 days. The present study has given no clear indication of apomixis in P. deltoides though the possibility has not been conclusively eliminated. Most seeds are produced as a result of pollination by fresh pollen resulting in normal fertilization.

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