Encyclopaedia Britannica, 11th Edition, Volume 2, Part 1, Slice 1 by Various
page 62 of 281 (22%)
page 62 of 281 (22%)
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also the synergidae. As in Gymnosperms and other groups an interesting
qualitative change is associated with the process of fertilization. The number of chromosomes (see PLANTS: _Cytology_) in the nucleus of the two spores, pollen-grain and embryo-sac, is only half the number found in an ordinary vegetative nucleus; and this reduced number persists in the cells derived from them. The full number is restored in the fusion of the male and female nuclei in the process of fertilization, and remains until the formation of the cells from which the spores are derived in the new generation. In several natural orders and genera departures from the course of development just described have been noted. In the natural order Rosaceae, the series Querciflorae, and the very anomalous genus _Casuarina_ and others, instead of a single macrospore a more or less extensive sporogenous tissue is formed, but only one cell proceeds to the formation of a functional female cell. In _Casuarina_, _Juglans_ and the order Corylaceae, the pollen-tube does not enter by means of the micropyle, but passing down the ovary wall and through the placenta, enters at the chalazal end of the ovule. Such a method of entrance is styled chalazogamic, in contrast to the porogamic or ordinary method of approach by means of the micropyle. _Embryology._ The result of fertilization is the development of the ovule into the seed. By the segmentation of the fertilized egg, now invested by cell-membrane, the embryo-plant arises. A varying number of transverse segment-walls transform it into a pro-embryo--a cellular row of which the cell nearest the micropyle becomes attached to the apex of the |
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