Most tissues of a plant are derived from meristems that reside at the tips of the shoot and root. To identify genes that are involved in the organization and in the coordination of cell division and differentation in the meristem, mutations that affect meristem development have been sought. Several mutations that affect meristem development have been characterized in Arabidopsis and maize; however, the angiosperm meristem is small, inconspicuous and difficult to characterize in a non-destructive way. Gametophytes of the fern Ceratopteris richardii have a meristem that is simple, small and conspicuous and is an ideal system for studying meristem development. In this study, the meristems of wild-type and various mutant gametophytes of Ceratopteris are described using cold stage SEM of hydrated gametophytes.

The haploid gametophytes of the fern Ceratopteris are either male or hermaphroditic and are easily distinguished by size and shape as well as by type of sex organ produced. Hermaphrodites form a distinct meristem; newly divided cells of this meristem have only four possible cells fates: they may differentiate as antheridia, the sperm-forming organs; differentiate as archegonia, the egg-forming organs; enlarge to contribute to the growing two-dimensional sheet of cells; or remain within to maintain the meristem (Fig. 1; an=antheridia; ar=archegonia). Male gametophytes lack a meristem and are thus much smaller than hermphrodites. Almost all cells of the male differentiate as antheridia, the sperm-forming organs (Fig. 2). Because the gametophytes of Ceratopteris are small (<3 mm), conspicious, free-living, and haploid, it is very simple to select mutations that affect the sex of the gametophyte as well the meristem.