Gibberellin activity was detected in vegetative Marchantia polymorpha. Increasing the photoperiod from 12 to 18 hours elicited quantitative differences in the activity of these gibberellins. Such changes were associated with increased thallus elongation and orthogeotropic growth in the 18-hour tissue. Gibberellin antagonists reduced the endogenous gibberellin levels and retarded both thallus elongation and erect growth. The physiological implications of this response and the role of gibberellins in thallus elongation are considered. The role of auxins in mediating geotropic curvature is widely recognized. Halevy et al. (1969), however, reported that gibberellin treatment caused certain varieties of peanut plants that possessed normally diageotropic branches to become orthogeotropic. The branch angle increased from a range of 0-25 degrees to one of 50-60 degrees. Such angles were similar to those normally found in peanut varieties that exhibited an erect growth habit. Furthermore, 2-(chloroethyl)-trimethyl-ammonium chloride (CCC) and 2-isopropyl-4-dimethylamino-5-methylphenyl-l-piperdine carboxylate methyl chloride (Amo 1618) inhibited this negative geotropic response. An analysis of the gibberellin content of the two different growth forms indicated that both quantitative and qualitative variations occurred. Recent experiments with sunflower (Phillips, 1972a) and corn (Phillips, 1972b) demonstrated that variations in endogenous gibberellin concentrations were associated with geotropism. In both species, the level of gibberellin transport was ten times greater in the ventral half of a horizontal shoot. Significant concentration differences were also associated with strong phototropic stimuli. Gibberellin levels were eight times higher on the shaded side of the shoot than on the lighted side. Fredricq and DeGreef (1968) observed that under short day conditions (12 hours light) thalli grew prostrate, but that when the light period was extended to 18 hours, the habit became erect. The physiological nature of this response was not apparent from their experimental data. There is some evidence that exogenous gibberellins can influence growth in bryophytes. Asprey et al. (1958) demonstrated that seta elongation in the sporophyte of Pellia epiphylla could be induced under short day conditions if the plant was treated with auxins or gibberellins. In fact treatment with either hormone, or with the two hormones simultaneously, elicited different responses. This suggests that gibberellin may promote auxin-mediated elongation, possibly through the 1Department of Biology, Lawrence University, Appleton, Wisconsin 54911. This content downloaded from 157.55.39.197 on Sun, 11 Dec 2016 04:34:27 UTC All use subject to http://about.jstor.org/terms 34 THE BRYOLOGIST [Volume 77