Pollen-grain sentence example

pollen-grain
  • The ANGIOSFERMS, which are much the larger class, derive their name from the fact that the carpel or carpels form a closed chamber, the ovary, in which the ovules are developedassociated with this is the development of a receptive or stigmatic surface on which the pollen grain is deposited.
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  • The pollen grain bears numerous spines, the dark spots indicate thin places in the outer wall.
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  • When placed on the stigma, under favourable circumstances, the pollen-grain puts forth a pollen-tube which grows down the tissue of the style to the ovary, and makes its way along the placenta, guided by projections or hairs, to the mouth of an ovule.
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  • Before following the growth of the pollen-grain after pollination, we will briefly describe the structure of a cycadean ovule.
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  • The number of chromosomes in the nucleus of the two spores, pollen-grain and embryo-sac, is only half the number found in an ordinary vegetative nucleus.
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  • The nucleus of the microspore divides and gives rise to a small cell within the large cell, a second small cell is then produced; this is the structure of the ripe pollen-grain in some conifers (Taxus, &c.).
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  • In the Abietineae cell-formation in the pollen-grain is carried farther.
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  • The extine is a firm membrane, which defines the figure of the pollen-grain, and gives colour to it.
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  • The surface of the pollen-grain is either uniform and homogeneous, or it is marked by folds formed by thinnings of the membrane.
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  • Within the pollen-grain is the granular protoplasm with some oily particles, and occasionally starch.
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  • Pollination having been effected, and the pollen-grain having reached the stigma in angio sperms or the summit of the nucellus in mnos erms P gY P it is detained there, and the viscid secretion from the glands of the stigma in the former case, or from the nucellus in the latter, induce the protrusion of the intine as a pollen-tube through the pores of the grain.
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  • In the light of our present knowledge of Ginkgo and the Cycads, there can scarcely be a doubt that spermatozoids were formed in the cells of the antheridium of the Cordaitean pollen-grain and that of other Palaeozoic Spermophyta; the an theridium is much more developed than in any recent Gymnosperm, and it may be doubted whether any pollen-tube was formed.
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  • As the tube grows, the contents of the pollen grain - essentially genetic material - passes down the tube.
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  • Pollen tube When a pollen grain lands on the stigma of a flower, it is stimulated to produce a pollen tube When a pollen grain lands on the stigma of a flower, it is stimulated to produce a pollen tube.
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  • Lower part of canal, enlarged; o, cavity of canal, surrounded by a sheath of cells, dilated towards the bottom of canal, in which a large pollen-grain is caught; ex, exterior of pollengrain; in, internal group of prothallial or antheridial cells.
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