The fruits of the elm, sycamore, and ash.—These common forest trees bear fruits with the seed attached to a flat plate, which is an outgrowth of the pericarp (p. 167). In the elm fruit (Fig. 124, 5) the plate is green and oval, and the seed forms a rounded swelling at, or near, its middle. The fruits of the sycamore generally grow in pairs (Figs. 33 and 137). Each half consists of a single seed with an attached membranous plate, and the two seed-boxes of each pair are in contact. The fruits of the ash hang from the twigs in bunches called “keys,” each fruit on a separate little stalk. The plates which bear the seed are long, narrow, and oval in shape. It is plain that such plates, or wings, expose a relatively large [Pg 174] surface to the air and prevent the fruit from falling to the ground as quickly as it otherwise would. The fruit is thus often blown to a great distance before it finally settles and the seed germinates. The action of the wing may be well shown by cutting off the plate from a sycamore fruit and letting it and one with an attached wing fall at the same instant from a height. The seed without a wing comes straight down as a pea would; but the winged seed spins in the air and settles more slowly. In the case of the round downy fruits of the lime, a similar service is performed by the bract (Fig. 126), upon which the flowers were carried. Pine and fir cones.—If a ripe pine, or fir cone is broken open, it will be seen that each seed is attached to a thin, papery wing (Fig. 130, 4), which has split off the upper surface of the scale bearing the seeds. The winged seeds are shaken out of the cone by the wind, and blown away. Trees alone bear winged seeds.—Winged seeds would be useless to any but fairly high trees, because if they were formed on the low plant they would fall to the ground long before the wind could catch them properly. It is also interesting to find that such seeds are generally attached so firmly that they are only broken off by gales strong enough to carry them a considerable distance. On the other hand, the tiny plumed fruit of the dandelion or thistle is so very light in comparison with the surface exposed to the air that it takes quite a long time to fall even a few inches. 37. SEEDS WHICH ARE SCATTERED BY ANIMALS. 1. Hooked fruits.—Examine plants of herb bennet (wood avens) in summer and autumn, and find the fruits. Brush your sleeve against the fruits, and notice how they cling to the cloth. Examine them with a lens and observe the hooks at the ends of the styles. [Pg 175] Compare goosegrass (cleavers) and find the hooks on the fruits.