WING SEEDS 67 
To pass on. Some seeds, many of them of con- 
siderable 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 seedanirregular 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 Composite, 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 Composite 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 
