This substance is found only upon the stamens or anthers of full-blown flowers (the anthers being the male organs), and is intended for the purpose of enabling the female portion of the flower to produce fertile seeds. In form the pollen grains are wonderfully diverse, affording an endless variety of beautiful shapes. In some cases the exterior is smooth and marked only with minute dots, but in many instances the outer wall of the pollen grain is covered with spikes, or decorated with stripes or belts. A few examples of the commonest forms of pollen will be found on Plate III. Fig. 17 is the pollen of the snowdrop, which, as will be seen, is covered with dots and marked with a definite slit along its length. The dots are simply tubercles in the outer coat of the grain, and are presumed to be formed for the purpose of strengthening the membrane, otherwise too delicate, upon the same principle which gives to “corrugated” iron such strength in proportion to the amount of material. Fig. 18 is the pollen of the wall-flower, shown in two views, and having many of the same characteristics as that of the snowdrop. Fig. 19 is the pollen of the willow-herb, and is here given as an illustration of the manner in which the pollen aids in the germination of plants. In order to understand its action, we must first examine its structure. All pollen-grains are furnished with some means by which their contents when thoroughly ripened can be expelled. In some cases this end is accomplished by sundry little holes called pores; in others, certain tiny lids are pushed up by the contained matter; and in some, as in the present instance, the walls are thinned in certain places so as to yield to the internal pressure. When a ripe pollen-grain falls upon the stigma of a flower, it immediately begins to swell, and seems to “sprout” like a potato in a damp cellar, sending out a slender “pollen-tube” from one or other of the apertures already mentioned. In Fig. 19 a pollen-tube is seen issuing from one of the projections, and illustrates the process better than can be achieved by mere verbal description. The pollen-tubes insinuate themselves between the cells of the stigmas, and, continually elongating, worm their way down the “style” until they come in contact with the “ovules.” By very careful dissection of a fertilised stigma, the beautiful sight of the pollen-tubes winding along the tissues of the style may be observed under a high power of the microscope. The pollen-tube is nothing more than the interior coat of the grain, very much developed, and filled with a substance technically named “fovilla,” composed of “protoplasm” (the semi-liquid substance which is found in the interior of cells), very minute starch grains, and some apparently oily globules.