An introduction to the study of heredity / by E.W. Macbride.

  • MacBride, E. W. (Ernest William), 1866-1940.
Date:
[1924]
    or oosphere surmounted by several cells situated in the neck of the flask, which break up into a kind of slime. When the male cells are ripe, the lid of their case breaks, they swarm forth and, swimming in the film of moisture underneath the prothallium, are attracted by the substances contained in the slime above the egg-cells. One spermatozoid reaches and fertilizes each egg-cell (the spiral portion of the spermatozoid which enters the egg-cell is the metamorphosed nucleus). As the egg-cells ripen later than the sper- matozoids, and usually many prothallia are found near each other, it will generally happen that a spermatozoid belonging to one pro- thallium will fertilize the egg-cell belonging to another prothallium. The fertilized egg cell then begins to divide and develops into a young fern plant, and as this grows the prothallium withers away. It follows that one prothallium can give rise to several fern plants. In the fern plant we are thus confronted with an alternation of generations. A genera tion which produces spores, or a spore-bearer (sporophore), is succeeded by a generation which produces egg-cells (oophore). A similar alternation of generations can be traced in all
    the higher plants, although modified and obscured, and we can trace a series of stages in this modification. Thus in certain water- ferns (Salviniacea) there are two kinds of spores produced, viz. small spores, termed micro spores, and large spores, termed macrospores. The sporangia containing these two varieties of spores are termed microsporangia and macro sporangia respectively. The microspores on germination give rise to prothallia which bear only spermatozoids, whereas the macrospores give rise to prothallia which produce only egg-cells. These may be termed the male and female prothallia respectively. The male prothallium sometimes consists of a single filament at the end of which two cells with granular contents are cut off. These cells constitute the antheridium, and in them the spermatozoids are developed (Fig. 5, A). The female prothallium (Fig. 5, B) consists of a mass of tissue projecting only a little way beyond the burst wall of the spore. In this mass are formed one or more archegonia, in each of which an egg-cell is developed. The macrospores, which produce female prothallia, are larger than the microspores, which produce male prothallia. Both macrospores and microspores are produced in fours by two
    successive divisions of a mother cell, but whereas in the formation of microspores all Illustrating the germination of the two kinds of spores of the water-fern (Salvinia). A. The germination of the microspore. 1. The spermatozoids in the vestigial antheridium. 2. The burst microspore. B. The germina tion of the macrospore. 1. The burst macrospore. 2. The vestigial prothallium. 3. The archegonium. 4. The egg-cell in the archegonium. 7. The wall of the burst macrosporangium. four sisters become spores, in the case of the macrospores only one of the four becomes a
    spore, whilst the other three cells remain small and remind us of the polar bodies in animal eggs. In that strange tropical plant the cycad there is essentially the same arrangement, but the female prothallium (termed the endosperm) remains entirely within the macrospore, only one of which is produced in each macrosporangium. The macrospore is now termed the embryo-sac. We find the further modification that the filamentous male prothallium is produced only when the microspore, or, as it is now termed, the pollen-grain, is wafted by the wind so as to alight on the macrosporangium, or, as it is now termed, the ovule. This male pro thallium consists of a filament termed the pollen-tube, at the apex of which the male germ-cells are formed, which, as in ferns, are spermatozoids, each provided with several flagella. In the pines and their allies the male cells are no longer typical spermatozoids, but small rounded naked nuclei, which when they escape, fall directly on the slime cells in the necks of the archegonia in the female prothallium. They easily traverse the slime and fertilize the egg-cells. Several young pine plants are thus formed in each embryo- B
    sac, but only one survives; the rest perish in consequence of the withdrawal of nourishment from them by the successful one (Fig. 6, A). A B Fig. 6. Longitudinal section through the “ ovules ” or macro sporangia of the seed-plants. A. Section through the ovule of a pine. 1. Macrospore or embryo-sac. 2. Endosperm or prothallium inside embryo-sac. 4. Arche- gonium. 5. Pollen-tube in male prothallium. 6. Male nucleus in pollen tube. 7. Macrosporangial wall. 8. Envelope surrounding macrosporangial wall. B. Section through the ovule of a flowering plant. 1. Embryo-sac. 4. Egg-cell and its two sister cells. 5. Antipodal cells. 6. Endosperm-forming cells. 7. Macrosporangial wall. 8. Inner envelope of macrosporangium. 9. Outer enve lope of macrosporangium. Finally, in ordinary flowering plants matters are very much as they are in the pines, but the macrosporangia or ovules are enclosed in a box or case formed by conjoined leaves