Influence of Photoperiod on Microsporogenesis in Cosmos sulphureus Cav. Var. Klondike

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Abstract
Exptl. groups of C. sulphureus var. Klondike were given photo-inductive cycles of 8 hrs. of light and 16 hrs. of darkness for 6, 7, 8, 10, 12, and 14 days. The groups were then divided into 3 lots each, which were given differential post-inductive photoperiods of 14, 19, or 24 hrs. Controls were run on natural and 8-hr. photoperiods; both of these lots flowered profusely. Three other controls run continuously on 14-, 19-, and 24-hr. photoperiods did not flower. The markedly suppressed buds of cosmos developing long foliaceous outer bracts and inner inhibited ones, showed no differentiation of floral pts. or sporogenous tissue. Less retarded reproductive structures produced floral tube, sporangial wall, a black-line tapetum, and sporogenous cells with little or no cytoplasm. These sporocytes did not initiate the prophases. In plants in which retardation was less marked, various abnormalities occurred, such as degeneration of tapetum, plasmodial masses derived from sporogenous tissues accompanying the normal stages, lagging chromosomes and the failure of some of them to be included in the micro-spores, and degenerating cytoplasm in the microspores. No pollen which appeared to be morphologically normal occurred in the anthers of flowers in which tapetal cells did not form a locular matrix. Although a quantitative record of buds of the exptl. lots clearly shows a reduction in total reproductive activity in relation to reduced photo-induction and to increased length of post-inductive photoperiod, it is cytologi-cally impossible to draw lines of demarcation between the lots.