Apex of Ovule, and Pollen-grain.

Before following the growth of the pollen-grain after pollination, we will briefly describe the structure of a cycadean ovule.

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.

- I, anther; 2, pollen grain of Hollyhock (Althaea rosea) enlarged.

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.

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.

This is remarkable in that it contains the first account of any value of the development of the pollen; as also a description of the structure of the pollen-grain, the confirmation of G.

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.

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.

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.

- I, anther; 2, pollen grain of Hollyhock (Althaea rosea) enlarged.

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.

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.

This is remarkable in that it contains the first account of any value of the development of the pollen; as also a description of the structure of the pollen-grain, the confirmation of G.

Before following the growth of the pollen-grain after pollination, we will briefly describe the structure of a cycadean ovule.

The pollen-grains find their way between the carpophylls, which at the time of pollination are slightly apart owing to the elongation of the internodes of the flower-axis, and pass into the pollen-chamber; the large cell of the pollen-grain grows out into a tube (Pt), which penetrates the nucellar tissue and often branches repeatedly; the pollen-grain itself, with the prothallus-cells, projects freely into the pollen-chamber (fig.

Pg, Pollen-grain.

A mature pollen-grain contains a prothallus of 3 to 5 cells (Fig.

13, Pg); the exine extends over two-thirds of the circumference, leaving a thin portion of the wall, which on collapsing produces a longitudinal groove similar to the median depression on the pollen-grain of a cycad.

After pollination the pollentube grows into the nucellar tissue, as in cycads, and the pollen-grain itself (fig.

Apex of Ovule, and Pollen-grain.

A pollen-grain when first formed from its mother-cell consists of a single cell; in this condition it may be carried to the nucellus of the ovule (e.g.

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.

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.

Sometimes the micropyle lies close to the base of the style, and then the pollentube enters it at once, but frequently it has to pass some distance into the ovary, being guided in its direction by various contrivances, as hairs, grooves, &c. In gymnosperms the pollen-grain resting on the apex of the nucellus sends out its pollen-tubes, which at once penetrate the nucellus (fig.

It was in this case that Renault first made the exceedingly interesting discovery that each pollen-grain contains a group of cells, presumably representing an antheridium (fig.

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.

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.

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.