Page:Popular Science Monthly Volume 39.djvu/353

Rh form four, each one surrounded by a cell-wall, but all still inclosed by the wall of the mother-cell (Fig. 1). On further growth the wall of the mother-cell is ruptured, and the daughter-cells escape as free pollen grains. The wall of the mother-cell is then either absorbed, or remains in the form of threads between the grains, or as viscid matter on the outside of the grains.

Thus pollen grains are daughter-cells, which have been developed from a primal mother-cell. Each grain is made up of three parts. There is an outer wall (extine), an inner wall (intine), and the fluid contents (fovilla). The extine is often marked with lines, points, or grooves; the intine is generally smooth and regular, and, even when the extine is studded with points, the intine does not



line the inside of these points but extends over their bases (Fig. 2). In cases where there are projections at different points, as in the evening primrose (Œnothera, Fig. 3) and others, the intine becomes thickened, and the extine is very much thinner. In the melon (Fig. 4), where the extine has pores provided with lids, the intine at these points is considerably thickened, and in growth pushes the cap off. When, again, a pollen grain appears marked with reticulations and spaces, as in Pancratium (Fig. 5), these are regarded as thinner places in the extine rather than special markings. It is a well-known fact that the pollen of anemophilous or wind-fertilized plants differs markedly from that of entomophilous or insect-fertilized plants. In the former case it is dry and powdery, probably having this quality from the entire absorption of the