Floral Morphology of Populus deltoides and P. tremuloides. Contributions from the Hull Botanical Laboratory 639 (1952)

1. Various aspects of the floral morphology of Populus deltoides, in which a time lag may occur between maturation of the staminate and pistillate catkins, are compared with those of P. tremuloides, in which this lag does not occur. 2. Floral buds are differentiated during the fall of the year preceding the opening of the flower. Stamens pass the winter in the spore-mother-cell stage. Nucellar primordia are conspicuous in winter pistillate buds, but the integuments do not appear until spring. In P. deltoides the number of carpels varies from two to four, while in P. tremuloides it is invariably two. The ovary is unilocular with parietal placentae. 3. The vascular anatomy of the staminate and pistillate flowers is very simple. Each stamen receives a single vascular bundle; each carpel receives the normal supply of vascular bundles-a dorsal and two ventral bundles. The bracts of both staminate and pistillate flowers are foliar in nature, and the vascular supply to the bract invariably passes off from the vascular cylinder in the pedicel as a single strand, leaving a single gap. In both staminate and pistillate 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. 4. Microsporogenesis follows the usual angiosperm pattern. The fully developed anther usually shows four layers of wall cells. The outermost consists of an endothecium with fibrous thickenings, and the innermost of a tapetum with cells usually binucleate at maturity. Before the two-nucleate pollen grains are shed, all wall layers between them and the endothecium disintegrate. Stomata occur on the connective and filaments in P. deltoides. 5. The ovules are anatropous, and each has a well-developed vascular strand. In P. deltoides the ovules are initially bitegmic, but in older ovules only one integument, the outer, can be distinguished. In P. tremuloides the ovules are unitegmic. 6. In the ovule archesporial cells are hypodermal in origin. In P. deltoides there are five or six, but usually only one develops. It gives rise to a parietal cell and a sporogenous cell. The latter becomes the megaspore mother cell which is rather deep-seated in the nucellus because of several divisions in the parietal cell. In P. tremuloides the archesporium is single-celled, and there are only two parietal cells between the nucellar epidermis and the megaspore mother cell. 7. The megaspore mother cell gives rise usually to a T-shaped, or occasionally to a linear, tetrad of megaspores. The chalazal spore is functional and develops into an eight-nucleate megagametophyte (Polygonum Type). During the later stages of development the megagametophyte projects through the nucellus into the micropyle. 8. The pollen tube reaches the megagametophyte through the micropyle. Double fertilization occurs. Division of the primary endosperm nucleus precedes that of the zygote and initiates an endosperm of the nuclear type, in which walls appear only at later stages. The first division of the zygote is always transverse to the long axis of the embryo sac. The first division of the embryo cell is always longitudinal, the second division transverse (Onagrad Type). 9. The "cotton" is composed of epidermal hairs of the placentae, not the seeds, and pieces of the placenta are shed with the seeds at maturity. 10. Certain variations of the usual pattern occur. More than one archesporial cell may develop, and thus two or more megagametophytes may be found in an ovule. Occasional flowers are found in which anther locules, or ovules, are degenerating. Some ovules may show certain or even all cells of the megagametophytes degenerating. Occasionally in P. tremuloides one to three staminate flowers occur near the base of the pistillate catkin. 11. Pollen germination tests in P. deltoides indicate that pollen grains readily germinate when they are fresh and that they are viable for only a short period of time. 12. 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. 13. The aspects of floral morphology reported correspond very closely to those reported for several species of Salix and Populus by other workers. 14. The Salicaceae give evidence of being a fairly highly evolved instead of a primitive family and should, therefore, occupy a higher position than that usually accorded to it by the Engler and Prantl system of classification. Within the family Populus is more primitive than Salix.