When spores of the fern, Onoclea sensibilis L., are allowed to germinate in darkness, the rhizoid and the protonema are positioned at close to a right angle. If the spores are exposed initially to light and allowed to germinate, the rhizoid and protonema are positioned nearly axially, at opposite ends of the spore. The greater the duration and intensity of the initial illumination, the greater the tendency towards axial arrangement. All colors of light are active to some degree, and the effects are intensity‐dependent. The response occurs in a uniform light field and is not dependent on a directional stimulus; the phenomenon reflects the relative arrangement of one part of the gametophyte to another part but not the orientation of growth with respect to an external stimulus. Direct tests show that neither the relative rhizoid orientation nor initial polarity of germination are affected by unilateral white light or polarized red light; the subsequent growth of the protonema, however, is oriented perpendicular to the plane of light polarization. The effects of light in determining the positional relationship between rhizoid and protonema are interpreted in terms of a hypothesis proposing light‐induced changes in the structure and mechanical properties of the spore wall.

Protonemata of Onoclea sensibilis and Diyopteris filix‐mas elongate in response to both red and far‐red light. The promotion caused by far‐red is larger than that caused by red light. This phenomenon differs from a typical response to phytochrome, the photoreceptor pigment immediately suggested by the activity of red and far‐red light. The phenomenon has been explained by two different hypotheses, one of which holds that phytochrome is solely responsible for the response, whereas the other postulates an interaction between phytochrome and P580, a yellow‐green light absorbing pigment, to account for the response. The hypothesis that phytochrome is the sole photoreceptor leads to some specific predictions concerning the shapes of the dose‐response curves for light‐induced protonema elongation. These predictions were tested with both continuous and short‐term irradiation. In all instances saturating far‐red light caused greater elongation than did saturating red light, and no dose of red light duplicated the activity of saturating far‐red light. Other experiments tested the interactions of red and far‐red light and the effects of different doses of yellow‐green light on protonema elongation. The results of many of the experiments were not in agreement with the hypothesis that phytochrome is the sole photoreceptor, whereas they were in agreement with the assumption that filament elongation is controlled by both phytochrome and P580.

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Germination in spores of the fern Onoclea sensibilis is initiated by an asymmetric division that partitions the spore into two cells of unequal size. The unequal daughter cells differentiate immediately into distinct types. When spores are germinated on the surface of solutions of methanol, the initial division is symmetrical, and the daughter cells from this equal division develop into the same type of cell. The differentiation of a rhizoid from the smaller cell in untreated spores is suppressed by methanol treatment.

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