Pollen-grain Sentence Examples
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.
The pollen grain bears numerous spines, the dark spots indicate thin places in the outer wall.
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.
Before following the growth of the pollen-grain after pollination, we will briefly describe the structure of a cycadean ovule.
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.Advertisement
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.).
In the Abietineae cell-formation in the pollen-grain is carried farther.
The extine is a firm membrane, which defines the figure of the pollen-grain, and gives colour to it.
The surface of the pollen-grain is either uniform and homogeneous, or it is marked by folds formed by thinnings of the membrane.
Within the pollen-grain is the granular protoplasm with some oily particles, and occasionally starch.Advertisement
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.
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.
As the tube grows, the contents of the pollen grain - essentially genetic material - passes down the tube.
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.
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.Advertisement