Why do the trees grow in such mixed groves, when Nature does the
planting? Here and there we find solid groves of beech or oak, but the
forest is, for the most part, a gathering together of all kinds of trees.
A part of the beauty of any woodland is this variety in the planting.
Under a tall oak we find a hornbeam, and under this the witch hazel, and
under the witch hazel, a carpet of low woodland plants. We may walk in a
straight line, or follow a woodland path a mile, and find every tree we
meet is different from all the rest.
Many reasons explain the order in which Nature plants forests. One of the
best of these is found in the kind of seeds trees bear. We shall find
that trees most widely scattered are those whose seeds are winged. It is
not hard to find, from May until far past midwinter, trees bearing light,
winged seeds. All through the summer, the wind is busy sowing the seeds
of the early-fruiting trees. In autumn, and all through the winter, the
sowing of the larger crop goes on.
Let us begin our study with the maples, whose winged seeds every child
knows. From the silver maple, whose seeds are dry before the first of
June, there is a procession of ripening maple seeds that lasts throughout
the year. A high wind shakes off the silver maple’s keys in showers in
late May. Watch those in the tree-tops. The wind has a better chance up
there. Each key, loosening from its twig, turns round and round in a
dizzy whirl, and sails away still whirling as it falls, the heavy seed
end always pointed downward. A tree is soon stripped, and the ground
littered under it. But a great deal larger area than the tree’s shadow
has the seeds scattered over it: the stronger the wind, the further these
seeds go. Before the summer is over, a crop of little maple trees springs
up from this sowing.
The red maple’s scarlet seed clusters turn brown, and the little winged
seeds take flight in June. Lighter and smaller, they are carried longer
distances than the seeds of the silver maple, and a crop of little red
maples follows this June sowing of the trees.
I remember walking in a corn field in late June; the corn had been last
ploughed a month before. Among the weeds that had grown up in this short
time was a crop of young red maples, now six inches high. It was amazing
to see these little trees grow so plentifully in a cultivated field. I
looked for the seed tree, and there it stood on the edge of the field,
the only maple tree in sight. A few young trees were growing in the
matted grass of the roadside under the tree, but the great crop was from
the seeds that flew out to the mellow ground between the corn rows. The
disappointed seeds, those which fell and did not grow, were under the
tree and in the dusty road.
In the autumn the hard maple, which we call the sugar maple, ripens its
winged seeds. So does the three-leaved box elder (which is a maple) and
the Norway maple, now a very familiar street tree. The wind takes its
time, and the trees stubbornly hang on to their seeds, so that these
maples are busy all winter with the sowing. Every day they give up a few,
and many seeds that fall on the snow are picked up, again and again, by
the wind and thus carried further and further away.
The maple seed, with its curiously one-sided wing, is the sign by which
the maple family is easily recognised. Other trees have winged seeds, but
none have the peculiar form of this one.
All summer long we may know the trees that belong to the ash family by
the clusters of pale green darts that hang among their leaves. These are
the ash seeds. Each one is a pointed seed case, containing the embryo
plant, and out behind it extends the thin, light, two-edged wing. There
is no one-sidedness to this blade. The seed is winged, but balanced like
a dart. When the wind loosens one from the wiry stem, it goes like an
arrow, seed downward. If there is a gale blowing, the seed may be caught
up and borne far away in the upper air, before a lull lets it take a
downward course, and drive its point into a snowbank, or into the ground.
This little feathered arrow may be long or short, depending upon whether
it belongs to the red ash, the white ash, or the black; but there is no
mistaking an ash tree for any other, once the form of an ash seed is
fixed in the mind.
I have said that a maple seed is shaped like that of no other tree. I
must describe here the seeds of the needle-leaved evergreens, which,
though very much smaller, are somewhat like maple seeds in form. Go to a
pine tree or a spruce, and get one of the cones that has begun to spread
its scales apart. Shake the cone over a piece of paper. If nothing comes
out from between the scales, cut or break the cone open with knife or
hatchet. Under each scale will be found two seeds, each with a thin,
one-sided wing. Spruces, hemlocks, firs, and arbor vitæs, all have this
same type of seed, hid away in the same fashion, under the protecting
scales of their cones. Do you understand how the wind, blowing through
the tops of evergreens, shakes the winged seeds from their places, and
carries them far away? Do you understand why the ripe cones of these
trees hang on so stubbornly, and spread their scales to allow the seeds
to escape?
It is a peculiarity of the firs that they hold their cones erect. It
would seem hard for the wind to get at the seeds, but the fir cones let
their scales fall, and when they loosen, the seeds are freed.
Out of the balls of the sweet gum tree, which dangle on the twigs all
winter, the wind shakes little winged seeds, not unlike those of the
pines.
Do you know the catalpa’s long, green pods that hang all summer on the
top of trees? They are longer than the newest lead pencil, and show no
signs of splitting, until the autumn. Now, the two halves of the pod
spread apart, and gradually the thin seeds shake out. Each one is in the
centre of a thin, fringed wing, that looks as if made of tissue paper.
The wind can carry these ghostly seeds for miles. Indeed, it is strange
that they ever come to the ground, for they seem to have no thickness nor
weight at all.
The birches all bear their seeds in cones, some long and pencil-like,
others quite the shape of a pine cone. Under each quaintly notched scale
of the cone, a seed is borne; and each heart-shaped seed has a thin rim,
which acts like a wing, catching the wind as the seed falls. We shall
look far in the woods before we find seeds daintier in form, or better
sailors through the air, than those of all the birch family.
The hop hornbeam has a hop-like cluster of seeds, each in an inflated
papery bag. When the leaves drop in the fall, the wind has a chance to
pick off these little paper seed balloons, one at a time, from the
clusters. Take off one of these little bags, open it, and you will find,
set in the bottom, the shiny, pointed seed. It is likely to have a long
journey, if there be a good breeze, before its bag is punctured.
Back to early May again, when the elm trees are green with their fruit
clusters, before the leaves are fully out. Elm trees grow scattered
through the woods, and no wonder: the seeds have papery rims, and the
wind catches these little falling discs, and scatters them far from the
tree where they were born.
The ailanthus tree, whose long, fern-like leaves make it look like a tree
from the Tropics, is sowing its seeds all winter, with the help of the
wind. Examine one. In the middle of a slim blade is the little seed. The
blade is twisted as it ripens, and it sails through the air with a
tilting, uncertain flight. After a look at a bunch of these seeds, and
after throwing a handful of them out of an upper window, and watching
them as they sail away, we shall understand how it is that ailanthus
trees spring up in most unexpected places, year after year. And we shall
bless the breeze that plants such trees along the hot pavements, and in
the ugly back alleys of towns and cities, where few trees are able to
grow at all.