. Morphology of gymnosperms. Gymnosperms; Plant morphology. 15° MORPHOLOGY OF GYMNOSPERMS. ary between the two becomes indistinct (fig. 177). Nothing is known of the behavior of chromatin during fertiHzation. The prominent development of the pollen tube as an absorbing organ suggests a question as to its original significance. In the Cycadofilicales the pollen grains are in close proximity to the egg, and there is nothing to indicate that any pollen tube was formed. In the cycads and in Ginkgo the pollen tube in the earlier stages of its development functions only as a haustorium. The pollen g
. Morphology of gymnosperms. Gymnosperms; Plant morphology. 15° MORPHOLOGY OF GYMNOSPERMS. ary between the two becomes indistinct (fig. 177). Nothing is known of the behavior of chromatin during fertiHzation. The prominent development of the pollen tube as an absorbing organ suggests a question as to its original significance. In the Cycadofilicales the pollen grains are in close proximity to the egg, and there is nothing to indicate that any pollen tube was formed. In the cycads and in Ginkgo the pollen tube in the earlier stages of its development functions only as a haustorium. The pollen grain end, with its cell complex, projects into the pollen chamber and gradually approaches the egg, not by actively invading any tissue, but merely by elongating into the enlarging pollen chamber. The portion which actively invades the nucellus is somewhat longer than the part in the pollen chamber, but has a much smaller diameter and never carries the sperms. In all other living seed plants, the pollen grain remains Just where it falls, and the pollen tube acts both as a haus- torium, actively invading the tissue, and as a sperm carrier. It seems safe to assume that the pollen tube was originally a haustorium, and that its function as a sperm carrier was developed later. 4. The embryo In 1877 Warming (8) described some of the later stages in the development of the embryo of Ceratozamia, but in all cases the embryo had already passed through the base of the egg and invaded the endosperm. In 1884 Treub (13) gave a comparatively full account of the embryogeny of Cycas circinalis, and, later, Ikeno (27) studied Cycas revoluta. Coulter and Chamberlain (33) investigated Zamia floridana (figs. 178-181) and more recently Chamberlain (70) described the embryogeny of Dioon edule (figs. 183-185) and Stangeria paradoxa (90), while Saxton gave an account of Enceph- alartos (76). Fig. 177.—Zamia flori- dana: fertilization; the smaller sperm nucleus has pressed into the egg nu- cleus, but
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