. Morphology of spermatophytes. [Part I. Gymnosperms]. Gymnosperms; Plant morphology. COMPARATIVE MORPHOLOGY OF GYMNOSPERMS 161 which passes over directly into a mother cell, and that the pecul- iar differentiation and breaking down of a zone of tissue about it has led to the impression of a sporogenous mass. In some cases where there is more than one mother cell, two or more megaspores begin to develop, and in Sequoia the sterile megaspores are prominent and persistent, but as a rule one soon becomes dominant at the expense of the rest and of the adjacent nucellar tissue. The mother cell incr
. Morphology of spermatophytes. [Part I. Gymnosperms]. Gymnosperms; Plant morphology. COMPARATIVE MORPHOLOGY OF GYMNOSPERMS 161 which passes over directly into a mother cell, and that the pecul- iar differentiation and breaking down of a zone of tissue about it has led to the impression of a sporogenous mass. In some cases where there is more than one mother cell, two or more megaspores begin to develop, and in Sequoia the sterile megaspores are prominent and persistent, but as a rule one soon becomes dominant at the expense of the rest and of the adjacent nucellar tissue. The mother cell increases in size in the usual way, and early in its history begins to germinate. The sequence of events seems to be very uniform, being free nuclear division (usually about eight simultaneous divisions), parietal placing of free nuclei in a cytoplasmic layer, organization of pari- etal tissue, and gradual filling of the embryo sac by centripetal growth. The cases of Tumboa and Gnetum are peculiar in that there is a distinct polarity in the tissues of the sac, and an appearance of reproductive cells in a sac not filled with tissue. In Tum- hoa the micropylar region of the sac con- tains cells but loosely aggregated when the reproductive cells appear, while in Gnetum the micropylar end of the sac, and in some species the whole sac, contains only free nuclei when fertilization occurs. Indica- tions of the same tendency in other forms are not lacking. In Sequoia it is reported that while cell formation is taking place at the two extremities of the sac, free nuclear division con- tinues in the middle region, the sac thus being distinctly divided into its reproductive and nutritive regions. In the development of the archegonia, which are variable in number and in distribution, there are some features that seem peculiar to the group. The neck canal cell series, which is by no means prominent in the Pteridophytes, is not represented at all in the Gymnosperms; and the ventral canal cell, c
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