. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. Fig Selaginella spinulosa, A. Br. Section of megasporangium showing the single fertile tetrad still very small, and the rest of the sporogenous cells arrested, x 100. Fit;. 52. [soetes lacuslris, L. Vertical section of a young microsporangium. .f/ = fertile tissue. tr= trabeculae. ^=tapetum. X too. while the remainder forms microspores. From the history of development,, and from comparison, the conclusion seems justified that the trabeculae and tapetum in this case represent sporogenous tissue which h
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. Fig Selaginella spinulosa, A. Br. Section of megasporangium showing the single fertile tetrad still very small, and the rest of the sporogenous cells arrested, x 100. Fit;. 52. [soetes lacuslris, L. Vertical section of a young microsporangium. .f/ = fertile tissue. tr= trabeculae. ^=tapetum. X too. while the remainder forms microspores. From the history of development,, and from comparison, the conclusion seems justified that the trabeculae and tapetum in this case represent sporogenous tissue which has been converted into sterile tissue, serving nutritive and mechanical purposes in the very large sporangium (Fig. 52). Similarly, in the megasporangium there is sterilisa- tion, but it has been carried much further, and it has been possible to show that the megaspore-mother-cells are not morphologically predetermined, but are physiologically selected from among a large number of potentially sporogenous cells: also that each archesporial cell gives rise to several megaspore-mother-cells, as well as to trabeculae and tapetum (Fig. 53) (Wilson Smith). Thus there has been a differentiation of tissues of uniform origin, and a large part has been diverted to functions played by sterile vegetative tissue. Very similar sterile tracts of tissue have been seen in the large sporangia of Lepidostfobus Brownii, and their origin by sterilisation is highly probable, though naturally this is hardly susceptible. 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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