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.
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.
The extine is a firm membrane, which defines the figure of the pollen-grain, and gives colour to it.
Apex of Ovule, and Pollen-grain.
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.).
Before following the growth of the pollen-grain after pollination, we will briefly describe the structure of a cycadean ovule.
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.
