To pass on. Some seeds, many of them of considerable size as compared with those which we have just considered, have coverings which are furnished with a membranous wing (Fig. 13, _d_), sometimes extending all round the seed, as in the Elm (_Ulmus_), more often placed at one side, as in the Sycamore (_Acer_). The effect of such wings is to reduce the rate of fall, imparting to the seed an irregular zigzag motion, as in the former case, or a spinning motion as in the latter. A Sycamore seed with the wing removed will fall four or five times as fast as with the wing present. But while a well-developed wing forms a more efficient dispersal device than mere reduction in size as found in Seed Plants, the rate of fall of wing seeds as a whole shows that these appendages do not fit them for anything but short voyages. We may then pass on to consider the plumed seeds, which possess by far the most efficient as well as the most beautiful devices for aiding dispersal found among wind-borne seeds. These plumed seeds belong to many different groups of plants, and the tufts of delicate hairs which give them their buoyancy arise in different ways. Among the _Compositæ_, the Order which furnishes the most familiar of our plumed seeds, the plume is formed by modification of the upper part of the calyx, which in so many common plants is small, green, and leaf-like; the lower part of the calyx in the _Compositæ_ is tough, persistent, and close-fitting, forming an additional protection for the seed. The plume springs either from the top of the seed, as in the Thistle, or is borne on a slender stalk, as in the Dandelion. It consists of a ring or radiating mass of hairs of beautiful delicacy, often bearing short [Illustration: FIG. 13.--WING-SEEDS AND PLUME-SEEDS. _a_, Mountain Willowherb (_Epilobium montanum_), 2/1; _b_, Dandelion (_Taraxacum officinale_), 2/1; _c_, Mountain Avens (_Dryas octopetala_), 1/1; _d_, Scotch Fir (_Pinus sylvestris_), 2/1; _e_, Reed-mace (_Typha latifolia_), 2/1.]