. Morphology of gymnosperms. Gymnosperms; Plant morphology. 328 MORPHOLOGY OF GYMNOSPERMS tinct enough to follow, and the enlarging mother cell is the first evi- dence of its existence. The number of mother cells varies as among the Pinaceae, but the only recorded cases of more than one mother cell are Taxus, in which there are one to five, and Podocarpus, in which there are occasionally two. In Taxus the occasional appear- ance of more than one developing embryo sac has been known ever since Hofmeister's investigations. An important fact in con- nection with the megasporangium of taxads is th
. Morphology of gymnosperms. Gymnosperms; Plant morphology. 328 MORPHOLOGY OF GYMNOSPERMS tinct enough to follow, and the enlarging mother cell is the first evi- dence of its existence. The number of mother cells varies as among the Pinaceae, but the only recorded cases of more than one mother cell are Taxus, in which there are one to five, and Podocarpus, in which there are occasionally two. In Taxus the occasional appear- ance of more than one developing embryo sac has been known ever since Hofmeister's investigations. An important fact in con- nection with the megasporangium of taxads is the general absence of any tapetal zone about the megaspore and embryo sac, so characteristic a feature of the Pinaceae. Among podocarps, however, "spongy tissue" occurs in Phyllocladus (174), Podocarpus (69), Dacrydium (174), and Saxegothaea, which indicates that it is of general occurrence in the tribe. The general lack of differ- entiation of any glandular cells in taxads may be regarded either as a primitive character or a degra- dation. The deep-seated megaspore mother cell usually forms a linear tetrad, the innermost megaspore functioning. As in Pinaceae, the second division may not occur in the upper daughter cell of the mother cell, resulting in the row of three cells originally reported for Taxus, and also in Phyllocladus (174); but complete tetrad- formation has now been observed in a sufficient number of cases to estabhsh it as the usual condition. Even the linear arrangement of the tetrad is occasionally departed from, as has been observed in Taxus (100). There are exceptions as to the functioning megaspore of a tetrad; while the innermost one usually functions, any one may function, and two contiguous ones have been observed to begin. Fig. 380.—Taxus baccata: three megaspore mother cells, each at the end of a row of cells derived from a primary tapetal cell; X37S.—^After Strasburger (100).. Please note that these images are extracted from scanned page i
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