SURVEY OF ANATOMICAL AND MICROCHEMICAL CHANGES IN THE SHOOT OF XANTHIUM PENNSYLVANICUM IN RELATION TO PHOTOPERIODISM

Abstract

IN THEIR GROWTH and development plants are affected by many environmental factors such as length of day, technically known as photoperiod (Garner and Allard, 1920). While the length of day may modify any and all parts of the plant, interest has been centered primarily on the initiation of inflorescence and flower primordia, and to a lesser degree on the development of flowers, fruits and seeds. The literature dealing with the process of flowering in vascular plants has been reviewed recently and comprehensively by Murneek (1948), Lang (1952) and Naylor (1953). The morphology of the developing inflorescence of Xanthium has been described in several studies (Mann, 1940; Naylor, 1941; Khudairi, 1953; Lockhart and Hamner, 1954; Lincoln, 1955; Salisbury, 1955) and in a few papers the anatomy has been outlined (Wilton and Roberts, 1936; Wilton, 1938; Hamner and Bonner, 1938; Struckmeyer, 1941; Roberts and Struckmeyer, 1948). Since the submission of this article for publication, a detailed account of shoot development has appeared (Millington and Fisk, 1956). The present study deals with the anatomical and microchemical changes which occur throughout the length of the stem in a period of about three weeks, as the Xanthium plants change from the vegetative to the reproductive state. MATERIALS AND METHODS.-Xanthium pennsylvanicum of the family Compositae is a monoecious annual well known in photoperiodic studies. It is a short-day plant, and the critical day length is about 15 hr. The flower heads are unisexual, the staminate uppermost, the pistillate below, bur-like. Plants of Xanthium pennsylvanicum were grown in the greenhouse at temperatures, in general, varying from 200 to 30?C. The burs were planted in flats and about two weeks later the seedlings were transplanted and were grown under long-day conditions of at least 18 hr. of light by supplementing the natural daylight with fluorescent lamps. The experiment was begun when the plants had three fully expanded leaves. Uniform plants were selected. Half the plants were placed under continuous short-day conditions where they received a photoperiod of 9 hr. of light and 15 hr. of darkness, the other half, which were used as controls, remained under long-day treatment. Internodes of