. Morphology of gymnosperms. Gymnosperms; Plant morphology. reports that four free nuclei (in one case six) pass to the bottom. There is agreement that the fusion nucleus of Taxodium passes to the bottom of the egg before division; while in Cunninghamia this position is taken at the two-nucleate stage, and in Sciadopitys at the four-nucleate stage (as in Pinus). The indications are that this movement occurs at any time before wall-formation, but most frequently after two divisions. In general, also, the completed pro- embryo consists of three tiers of cells instead of the four observed in Pinu
. Morphology of gymnosperms. Gymnosperms; Plant morphology. reports that four free nuclei (in one case six) pass to the bottom. There is agreement that the fusion nucleus of Taxodium passes to the bottom of the egg before division; while in Cunninghamia this position is taken at the two-nucleate stage, and in Sciadopitys at the four-nucleate stage (as in Pinus). The indications are that this movement occurs at any time before wall-formation, but most frequently after two divisions. In general, also, the completed pro- embryo consists of three tiers of cells instead of the four observed in Pinus, the ''rosette" tier not being present. The tiers may not contain the same number of cells, as the proembryo of Taxodium will illus- trate. Preceding wall-formation the eight nuclei are arranged in two tiers, the upper containing six cells, the lower two. There is only one division of tiers after wall-for- mation, which divides the upper tier into the suspensor cells and the open cells; at the same time, the two basal cells divide by vertical walls to a tier of four cells. The completed proembryo, therefore, consists of six open cells, six sus- pensor cells, and four embryo cells. The case of Sequoia sempervirens (92) is very peculiar among Pinaceae. There is no" free nuclear division at all, a wall being formed at the first division, which results in two cells nearly filling the egg (fig. 356). Succeeding divisions result in a linear row of five large cells, the lowest forming the embryo, and the next upper one elongating to form the sus- pensor. The elimination of the free nuclear stage and the complete occupation of the egg by a few embryonal cells are facts difiicult to relate at present. Fig. 356.âSequoia sem- pervirens: first division of the fertilized egg, showing a wall forming between the two daughter nuclei; Xsoo. â âAfter Lawson (92).. Fig. 357.âSciadopitys verlicillata: secondary sus- pensors.âAfter Aenoldi (64).. Please note that these images are e
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