[Illustration: Fig. 341. Section of staminate cone, showing sporangia.]
[Illustration: Fig. 342. Two sporophylls removed, showing opening of sporangia.]
[Illustration: Fig. 343. Pollen grain of white pine.]
=612. Microspores of the pine, or pollen grains.=—A mature pollen grain of the pine is shown in fig. 343. It is a queer-looking object, possessing on two sides an air sac, formed by the upheaval of the outer coat of the spore at these two points. When the pollen is mature, the moisture dries out of the scale (or stamen, as it is often called here) while it ripens. When a limb, bearing a cluster of male cones, is jarred by the hand, or by currents of air, the split suddenly opens, and a cloud of pollen bursts out from the numerous anther locules. The pollen is thus borne on the wind and some of it falls on the female flowers.
[Illustration: Fig. 344. White pine, branch with cluster of mature cones shedding the seed. A few young cones four months old are shown on branch at the left. Drawn from photograph.]
[Illustration: Fig. 345. Mature cone of white pine at time of scattering of the seed, nearly natural size.]
=613. Form of the mature female cone.=—A cluster of the white pine cones is shown in fig. 344. These are mature, and the scales have spread as they do when mature and becoming dry, in order that the seeds may be set at liberty. The general outline of the cone is lanceolate, or long oval, and somewhat curved. It measures about 10-15_cm_ long. If we remove one of the scales, just as they are beginning to spread, or before the seeds have scattered, we shall find the seeds attached to the upper surface at the lower end. There are two seeds on each scale, one at each lower angle. They are ovate in outline, and shaped somewhat like a biconvex lens. At this time the seeds easily fall away, and may be freed by jarring the cone. As the seed is detached from the scale a strip of tissue from the latter is peeled off. This forms a “wing” for the seed. It is attached to one end and is shaped something like a knife blade. On the back of the scale is a small appendage known as the cover scale.
[Illustration: Fig. 346. Sterile scale. Seeds undeveloped.]
[Illustration: Fig. 347. Scale with well-developed seeds.]