296. =Pollen.= This is the powdery matter, commonly of a yellow color, which fills the cells of the anther, and is discharged during blossoming, after which the stamens generally fall or wither away. Under the microscope it is found to consist of grains, usually round or oval, and all alike in the same species, but very different in different plants. So that the plant may sometimes be recognized from the pollen alone. Several forms are shown in the accompanying figures. [Illustration: Fig. 309. Magnified pollen of a Lily, smooth and oval; 310, of Echinocystis, grooved lengthwise; 311, of Sicyos, with bristly points and smooth bands; 312, of Musk Plant (Mimulus), with spiral grooves; 313, of Succory, twelve-sided and dotted.] 297. An ordinary pollen-grain has two coats; the outer coat thickish, but weak, and frequently adorned with lines or bands, or studded with points; the inner coat is extremely thin and delicate, but extensible, and its cavity when fresh contains a thickish protoplasmic fluid, often rendered turbid by an immense number of minute particles that float in it. As the pollen matures this fluid usually dries up, but the protoplasm does not lose its vitality. When the grain is wetted it absorbs water, swells up, and is apt to burst, discharging the contents. But when weak syrup is used it absorbs this slowly, and the tough inner coat will sometimes break through the outer and begin a kind of growth, like that which takes place when the pollen is placed upon the stigma. [Illustration: Fig. 314. Magnified pollen of Hibiscus and other Mallow-plants, beset with prickly projections; 315, of Circæa, with angles bearing little lobes; 316, of Evening Primrose, the three lobes as large as the central body; 317, of Kalmia, four grains united, as in most of the Heath family; 318, of Pine, as it were of three grains or cells united; the lateral empty and light.] 298. Some pollen-grains are, as it were, lobed (as in Fig. 315, 316), or formed of four grains united (as in the Heath family, Fig. 317): that of Pine (Fig. 318) has a large rounded and empty bladder-like expansion upon each side. This renders such pollen very buoyant, and capable of being transported to a great distance by the wind. 299. In species of Acacia simple grains lightly cohere into globular pellets. In Milkweeds and in most Orchids all the pollen of an anther-cell is compacted or coherent into one mass, called a _Pollen-mass_, or POLLINIUM, plural POLLINIA. (Fig. 319-322.) [Illustration: Fig. 319. Pollen, a pair of pollinia of a Milkweed, Asclepias, attached by stalks to a gland; moderately magnified.] [Illustration: Fig. 320. Pollinium of an Orchis (Habenaria), with its stalk attached to a sticky gland; magnified. 321. Some of the packets or partial pollinia, of which Fig. 320 is made up, more magnified.] [Illustration: Fig. 322. One of the partial pollinia, torn up at top to show the grains (which are each composed of four), and highly magnified.]