Take a pair of sycamore fruits. Cut off the wing from one and let it and an uninjured fruit fall at the same instant from a height. Which reaches the ground first? Why?
4. Pine and fir cones.—Examine and compare pine and fir cones of various ages. Break open a ripe cone and see the scales with the pairs of naked seeds, each seed bearing a thin, papery wing which has split off from the upper surface of the scale.
Fig. 136.—“Clock”
of Dandelion
Fruits. (× ¼.)
Wind-sown seeds.—Many plants depend on the wind for the dispersal of their seeds, and consequently the seeds are provided with outgrowths of various kinds, which increase the surface greatly without adding much to the weight, and, acting like parachutes, offer increased resistance to the air and thus prevent the seeds from falling quickly to the ground. In some cases the outgrowth is part of the pericarp, in others it is an appendix carried by the seed itself, while in the lime it is the bract upon which the flowers were formerly borne.
The dandelion fruit.—The fruit of the dandelion ([Fig. 136]) affords one of the best possible examples of wind-dispersal. It will be remembered ([p. 113]) that what is commonly called the flower of the dandelion is really a head of perhaps 300 complete flowers: each with a hairy calyx-tube, a yellow, strap-shaped corolla, five stamens, and a pistil. When the flowers have been fertilised, the yellow corollas and the stamens wither, the ovary increases in size with the ripening of the single nutlet in its interior, and each calyx-tube elongates until it is about an inch in length, the tuft of fine hairs being still at its upper end. The attachment of the fruit ([Fig. 86], 4) to the disc (receptacle) is so slight when the seed is ripe that a very gentle puff of air is sufficient to overcome it. The tuft of hairs at the upper end has by this time expanded until it acts like a parachute, which supports the tiny fruit for a long time in the air.
The common thistle—a relative of the dandelion—also distributes its fruit by means of a tuft of fine hairs derived from the calyx. In this case the hairs radiate from the seed. Such “thistle down” is commonly found floating through the air in summer.
