. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. CASE OF PSILOTUM 89 group have a common origin, and as sister cells they develop alike at first: they may therefore be held to be equipotential cells (Fig. 45 c). The conclusion that they are so is supported by comparison with plants having some degree of affinity with Psilotum, such as Lycopodium: here all the cells of a sporogenous group essentially similar to that of Psilotum are equipotential; all of them normally undergo the tetrad-division. Fig. 45. Psilotum triquetrum, Sw. Various stages of dev
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. CASE OF PSILOTUM 89 group have a common origin, and as sister cells they develop alike at first: they may therefore be held to be equipotential cells (Fig. 45 c). The conclusion that they are so is supported by comparison with plants having some degree of affinity with Psilotum, such as Lycopodium: here all the cells of a sporogenous group essentially similar to that of Psilotum are equipotential; all of them normally undergo the tetrad-division. Fig. 45. Psilotum triquetrum, Sw. Various stages of development of the synangium and sporangium. In C the sporogenous group is shaded. D shows the differentiation of its cells, the fertile cells being shaded. E shows the disorganisation of the remaining cells without forming spores. X 100. and develop spores. But in Psilotum the' originally uniform group differentiates at a relatively late period into fertile and sterile cells, the former having dense protoplasm, and undergoing the tetrad-division; the latter having less dense protoplasm (Fig. 45 d) : these become dis- organised without tetrad-division, and their substance goes to nourish the young spores (Fig. 45 e). The conclusion to be drawn is that in Psilotum all the cells of the sporogenous mass are potentially sporogenous, as they are in Lycopodium, and probably were actually fertile in ancestral forms: that some of them have been sterilised, that is, diverted from. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bower, F. O. (Frederick Orpen), 1855-1948. London, Macmillan and Co. , Ltd.
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