SEEDS TRANSPORTED BY WIND.
17. How pigweeds get about.--In winter we often see dead tops of
lamb's-quarters and amaranths--the smooth and the prickly
pigweeds--still standing where they grew in the summer. These are
favorite feeding grounds for several kinds of small birds, especially
when snow covers the ground.
Many of the seeds, while still enclosed in the thin, dry calyx, and
these clustered on short branches, drop to the snow and are carried
off by the wind. Notwithstanding the provision made for spreading
the seeds by the aid of birds and the wind, the calyx around each
shiny seed enables it to float also; when freed from the calyx, it
drops at once to the bottom. Many kinds of dry fruits and seeds in
one way or another find their way during winter to the surface of
the ice-covered rivers. When the rivers break up, the seeds are
carried down stream, and perhaps left to grow on dry land after the
water has retired. Most of the commonest plants, the seeds of which
are usually transported by water, are insignificant in appearance
and without common names, or with names that are not well understood.
This is one reason for omitting the description of others which are
ingeniously fitted in a great variety of different ways for traveling
by water.
18. Tumbleweeds.--Incidentally, the foregoing pages contain some
account of seeds and fruits that are carried by the aid of wind, in
connection with their distribution by other methods; but there are
good reasons for giving other examples of seeds carried by the wind.
There is a very common weed found on waste ground and also in fields
and gardens, which on good soil, with plenty of room and light, grows
much in the shape of a globe with a diameter of two to three feet.
It is called _Amaranthus albus_ in the books, and is one of the most
prominent of our tumbleweeds. It does not start in the spring from
seed till the weather becomes pretty warm. The leaves are small and
slender, the flowers very small, with no display, and surrounded by
little rigid, sharp-pointed bracts. When ripe in autumn, the dry,
incurved branches are quite stiff; the main stem near the ground
easily snaps off and leaves the light ball at the mercy of the winds.
Such a plant is especially at home on prairies or cleared fields,
where there are few large obstructions and where the wind has free
access.
The mother plant, now dead, toiled busily during the heat of summer
and produced thousands of little seeds. The best portion of her
substance went to produce these seeds, giving each a portion of rich
food for a start in life and wrapping each in a glossy black coat.
Now she is ready to sacrifice the rest of her body to be tumbled about,
broken in pieces, and scattered in every direction for the good of
her precious progeny, most of whom will find new places, where they
will stand a chance the next summer to grow into plants. Sometimes
the winds are not severe enough or long enough continued, and these
old skeletons are rolled into ditches, piled so high in great rows
or masses against fences that some are rolled over the rest and pass
on beyond. Occasionally some lodge in the tops of low trees, and many
are entangled by straggling bushes. In a day or two, or in a week,
or a month, the shifting wind may once more start these wrecks in
other directions, to be broken up and scatter seeds along their
pathway.
During the Middle Ages in southern Egypt and Arabia, and eastward,
a small plant, with most of the peculiarities of our tumbleweed just
described, was often seen, and was thought to be a great wonder. It
was called the "rose of Jericho," though it is not a rose at all,
but a first cousin to the mustard, and only a small affair at that,
scarcely as large as a cabbage head. A number of other plants of this
habit are well known on dry plains in various parts of the world;
one of the most prominent in the northern United States is called
the Russian thistle, which was introduced from Russia with flaxseed.
In Dakota, often two, three, or more grow into a community, making
when dry and mature a stiff ball two to three feet or more in diameter.
[Illustration: FIG. 25.--Mature dry plant of Russian thistle as a
tumbleweed. (One-seventh natural size.)]
One of our peppergrasses, _Lepidium intermedium_, sometimes attains
the size and shape of a bushel basket; when ripe, it is blown about,
sowing seeds wherever it goes. The plants of the evening primrose
sometimes do likewise, also a spurge, _Euphorbia_ [_Preslii_]
_nutans_, a weed a foot to a foot and a half high.
Low hop clover, an annual with yellow flowers, which has been
naturalized from Europe, has developed recently on strong clay land
into a tumbleweed six inches in diameter. The tops of old witch grass,
_Panicum capillare_, and hair grass, _Agrostis hyemalis_, become
very brittle when ripe, and snap from the parent stem and tumble about
singly or in masses, scattering seeds by the millions. I have seen
piles of these thin tops larger than a load of hay where they had
blown against a grove of trees, and in some cases many were caught
in the tops of low trees.
[Illustration: FIG. 26.--The top of old witch grass as a tumbleweed.
(Reduced two thirds.)]
Bug seed and buffalo bur are tumbleweeds. In autumn the careful
observer with an eye to this subject will be rewarded by finding many
other plants that behave more or less as tumbleweeds. Especially is
this the case on prairies. These are annuals, and perish at the close
of the growing season. There are numerous other devices by which seeds
and fruit secure transportation by the wind.
19. Thin, dry pods, twisted and bent, drift on the snow.--The common
locust tree, _Robinia Pseudacacia_, blossoms and produces large
numbers of thin, flat pods, which remain of a dull color even when
the seeds are ripe. The pods of the locust may wait and wait, holding
fast for a long time, but nothing comes to eat them. They become dry
and slowly split apart, each half of the pod usually carrying every
other seed. Some of the pods with the seeds still attached are torn
off by the wind and fall to the ground sooner or later, according
to the force of the wind. Each half-pod as it comes off is slightly
bent and twisted, and might be considered a "want-advertisement"
given to the wind: "Here I am, thin, dry, light and elastic, twisted
and bent already; give me a lift to bear these precious seeds up the
hill, into the valley, or over the plain."
[Illustration: FIG. 27.--Two views of a half-pod of common locust,
dry, twisted, and bent, ready for a breeze.]
And the wind is sure to come along, a slight breeze to-day tossing
the half-pod a few feet, leaving it perhaps to be again and again
moved farther forward. The writer has seen these half-pods
transported by this means more than a block. But many of the pods
stick to the limbs till winter comes. Then a breeze tears off a few
pods and they fall on the snow, which has filled up all the crevices
in the grass and between the dead leaves and rubbish. Each half-pod,
freighted with every other seed, is admirably constructed; like an
ice boat, it has a sail always spread to the breeze. In this way there
is often nothing to hinder some of the seeds from going a mile or
two in a few minutes, now and then striking some object which jars
off a seed or two. The seeds are very hard, and no doubt purposely
so, that they may not be eaten by insects or birds; but once in moist
soil, the covering slowly swells and decays, allowing the young plant
to escape. Thus the locust seeds are provided with neither legs, wings,
fins, nor do they advertise by brilliant hue and sweet pulp; but they
travel in a way of their own, which is literally on the wings of the
wind.
20. Seeds found in melting snowdrifts.--It will interest the student
of nature to collect a variety of seeds and dry fruits, such as can
be found still on the trees and other plants in winter, and try some
of them when there is snow on the ground and the wind blows, to see
how they behave. Again, when the first snow banks of the early winter
are nearly gone, let him collect and melt a quantity of snow and search
for seeds. By this means he can see, as he never saw before, how one
neighbor suffers from the carelessness of another.
21. Nuts of the basswood carried on the snow.--Here are some notes
concerning the distribution of the spherical nuts of basswood. The
small clusters of fruit project from a queer bract which remains
attached before and after falling from the tree.
[Illustration: FIG. 28.--Fruit and bract of basswood well adapted
for moving before the wind on the snow.]
This bract, when dead, is bent near the middle and more or less twisted,
with the edges curving toward the cluster of nuts. From two to five
nuts about the size of peas usually remain attached till winter, or
even a few till spring. This bract has attracted a good deal of
attention, and for a long time everybody wondered what could be its
use. We shall see. The cluster of nuts and the bract hang down,
dangling about with the least breath of wind, and rattling on the
trees because the enlarged base of the stem has all broken loose
excepting two slender, woody threads, which still hold fast. These
threads are of different degrees of strength; some break loose after
a few hard gales, while others are strong enough to endure many gales,
and thus they break off a few at a time. The distance to which the
fruit can be carried depends on the form of the bract, the velocity
of the wind, and the smoothness of the surface on which the fruit
falls. When torn from the tree the twist in the bract enables the
wind to keep the cluster rapidly whirling around, and by whirling
it is enabled to remain longer suspended in the air and thus increase
the chances for a long journey. In throwing some of these from a
third-story window, it was found that a bract with no fruit attached
would reach the ground sooner than a bract that bore from two to four
solid nuts. The empty or unloaded bracts tumble and slide through
the air endwise, with nothing to balance them or steady their descent,
while the fruit on other bracts holds them with one side to the air,
which prolongs their descent. The less a loaded bract whirls, the
faster its descent, and the more a bract whirls when the wind blows,
the farther it is carried. The bract that is weighted with a load
of fruit acts as a kite held back by a string, and when in this position
the wind lifts the whole as well as carries it along. Before snow
had fallen in 1896, by repeated moves on a well-mowed lawn, fruit
and bracts were carried about two hundred feet, while with snow on
the ground the distance was almost unlimited, excepting where there
were obstructions, such as bushes and fences. When there is a crust
on the snow and a good wind, the conditions are almost perfect. Over
the snow the wind drives the bracts, which drag along the branch of
fruit much as a sail propels a boat. The curving of the edges of the
bract toward the fruit enables the wind to catch it all the better,
and to lift it more or less from the snow. With changes in the direction
of the wind, there is an opportunity for the fruit of a single tree,
if not too much crowded by others, to spread in all directions. After
watching these maneuvers, no one could doubt the object of the bent
bracts of the basswood, and as these vary much in length and width
and shape on different trees, it would seem that perhaps nature is
still experimenting with a view to finding the most perfect structure
for the purpose.
About one hundred and thirty paces west of the house in which I live
stand two birch trees. One windy winter day I made some fresh tracks
in the snow near my house, and within a few minutes the cavities looked
as though some one had sprinkled wheat bran in them, on account of
the many birch seeds there accumulated.
Other fruits in winter can be experimented with, such as that of box
elder, black ash, birches, tulip tree, buttonwood, ironwood, blue
beech, and occasionally a maple.
22. Buttonwood balls.--Nature seems to have no end of devices for
sowing seeds to advantage. Here is one which always interests me.
The fruit of the buttonwood, or sycamore, which grows along streams,
is in the form of balls an inch and a half in diameter. These balls
grow on the tops of the highest branches, and hold on into winter
or longer. The stems are about two inches long, and soon after drying,
through the action of the winds, they become very flexible, each
resembling a cluster of tough strings. The slightest breeze moves
them, and they bob around against each other and the small branches
in an odd sort of way. After so much threshing that they can hold
no longer, the little nuts become loosened and begin to drop off a
few at a time. Certain birds eat a few and loosen others, which escape.
The illustration shows some of these nuts, each supplied with a ring
of bristles about the base, which acts as a parachute to permit the
wind the easier to carry them for some distance before falling, or
to drift them on the surface of the snow or ice.
[Illustration: FIG. 29.--(a) Lax stem, supporting (b) fruit of the
buttonwood, or sycamore; (c) a single fruit separated, ready for the
wind or water.]
23. Seeds that tempt the wind by spreading their sails.--On low lands
in the cool, temperate climate of Europe, Asia, and North America,
is a common plant here known as great willow-herb, a kind of fireweed
(_Epilobium angustifolium_). There are several kinds of fireweeds.
This one grows from three to five feet high, and bears pretty pink
flowers. In mellow soil the slender rootstocks spread extensively,
and each year new sprouts spring up all around, six to eight feet
distant. Below each flower ripens a long, slender pod, which splits
open from the top into four parts, that slowly curve away from a
central column. The apex of each seed is provided with a cluster of
white silky hairs nearly half an inch long.
[Illustration: FIG. 30.--Fruit of willow-herb exposing seeds for
distribution by the wind.]
The tips of the hairs stick slightly to the inside of the recurved
valves, some hairs to one valve, and often others to the adjacent
valve, thus spreading them apart with the seed suspended between.
Four rows of the seeds are thus held out at one time. Often not over
half, or even a tenth part, of the seeds are well developed, yet the
silky hairs are present and float away in clusters, thus helping to
buoy those that are heavy. This is a capital scheme, for when the
pods are dry and unfurled, they silently indicate to the slightest
breath of air that they are ready for a flight, and it doesn't take
much to carry them for a long distance. As an active boy delights
to venture again and again over thin ice on a shallow pond in the
pasture, half fearing, yet half hoping, that he may become a hero
by breaking through and escaping, so likewise many of these seeds
and seed-like fruits spread themselves out, as if to tempt the wind
to come along and attack them.
The twin fruits of the parsnip and some of its near relatives are
light and thin and split apart, each holding on lightly to the top
of a slender stem. In this position they are sure to be torn off sooner
or later. Somewhat after the manner of the willow-herb behave the
pods and seeds of willows, poplars, milkweeds, Indian hemp, and
cotton.
[Illustration: FIG. 31.--Dry twin fruits of the parsnip held by
slender stems ready to be blown away. (Much enlarged.)]
24. Why are some seeds so small?--Do you know why so many kinds of
plants produce very small and light seeds? Would it not be better
if they produced fewer and larger seeds, which would then be stronger
and better able to grow under adverse conditions? But a large number
of small seeds cost the plant no more effort than a small number of
large ones, and the lighter and smaller the seeds and the more there
are of them, the better their chances for distribution, especially
for long distances. The minute size of spores of most of the fungi
are given as reasons why so many of them are so widely distributed.
Why is a boy or man of light weight chosen to ride the horse on the
race track? That the animal may have less weight to carry and thereby
use his surplus strength in making better time. The less weight the
parachute of the seed of the willow-herb has to carry, the greater
the chances for success in making a long journey. Of the willow-herb
it takes one hundred seeds to weigh a milligram, including the hairs
attached to them, and it would take thirty thousand to weigh as much
as an ordinary white bean.
25. Seeds with parachutes.--Many years ago large portions of Huron
and Sanilac counties of eastern Michigan were swept by a fire so severe
that the timber was all killed. Fifteen years later the woody growth
consisted mostly of willows, poplars, and birches. The seeds of all
kinds of willows and poplars are very light, and are produced in
immense quantities. Like those of the great willow-herb, they are
beautifully constructed for making long journeys through the air--a
fact that explains the frequency of these trees in burned districts.
A considerable number of seeds and fruits grow with a parachute
attached at one end, not to prevent injury by falling from the tree
top, but to enable the wind to sustain and transport them for a longer
distance.
26. A study of the dandelion.--In spring the dandelion is almost
everywhere to be found; every one knows it--the child to admire, the
gardener to despise. From each cluster of leaves spreading flat in
the grass come forth several hollow stems, short or tall, depending
on the amount of sunshine and shade. Each stem bears, not one flower,
but a hundred or more small ones. Around and beneath each yellow
cluster are two rows of thin, green, smooth scales (involucre).
[Illustration: FIG. 32.--Heads of the dandelion in fruit, closed and
open.]
The short outer row soon curls back, as though for rest or ornament,
or for watching the progress of the colony above; but the inner row
has a very important duty yet to perform in guarding the large family
within. At night, or in daytime, if the day be wet, the long scales
press like a blanket closely about the flowers, and do not permit
them to come out; but when the sun is bright, it shrinks the outer
side of these scales, which then curl apart, leaving the yellow
flowers ready for bees to visit or boys to admire and study. For
several days the flowers of a head blossom in succession, each night
to be snugly wrapped by the scales, and the next day to be again left
open, if the weather be fine. After each flower in turn has been
allowed to see the light, and after all have been crawled over by
bee and wasp to distribute the yellow pollen that seeds may be produced,
there is nothing else to do but patiently wait for a week or two while
receiving food from the mother plant to perfect each little fruit
and seed. During all this period of maturing, day and night, rain
or shine, the scales hold the cluster closely; the stem bends over
to one side, and the rain and dew is kept from entering. After a while,
on some bright morning, the dandelion stalk is seen standing erect
again, and is probably surrounded by many others in a similar position.
The dry air shrinks the outside of the scales, and they turn downward;
the circle of feathers at the top of the slender support attached
to the seed-like fruit below spreads out, and the community, which
now looks like a white ball of down, is ready for a breeze. The feathery
top is now ready to act as a parachute, and invites the wind to catch
up the whole and float it away. If there is no breeze, the moist air
of night closes the outer scales; each of the feathery tips closes,
and all are secure till the next bright day.
Of a like nature are fruits of thistles, fireweed, prickly lettuce,
sow thistles, scabiosa, valerian, cat-tail flag, cotton grass, some
anemones, smoke tree, virgin's bower, and some of the grasses.
27. How the lily sows its seeds.--Ripened pods of lilies usually stand
straight up on a stiff, elastic stem; beginning at the top, each one
slowly splits into three parts, which gradually separate from each
other. Why do they not burst open all of a sudden, like pea pods,
and shoot the seeds all about and have the job done with? Or why does
not the pod burst open at the lower end first, instead of the upper?
[Illustration: FIG. 33.--At the left a dry fruit of a lily opening
to permit seeds to dry and the wind to enter; to the right, a fruit
later in the season. Two views of a flat seed.]
Observe that the three opening cells are lashed together loosely with
a latticework. No slight breeze can dislodge the seeds, but just see
how they behave in a good gale! The elastic stems are swayed back
and forth against each other, and some of the upper seeds are tossed
out by the wind that passes through the lattice, and at such times
are often carried to some distance. The seeds at the top having escaped,
the dry pods split down farther and still farther and open still wider,
till the bottom is reached. As the seeds are not all carried away
the first or even the second time, and as succeeding breezes may come
from different directions, it is thus possible for the lily to scatter
its seeds in all directions.
The seeds of the lily are flat, very thin, and rather light, not
designed to be shot out like bullets, but to be carried a little way
by the wind; the pods are erect, and open at the top, that the seeds
need not escape when there is no wind or unless some animal gives
the stem a strong shake. The latticework was made for a purpose, and
the gradual opening of the pods prevents the supply from all going
in one direction or in one day, for a better day may arrive. The student
will look for and compare the following: Iris, figwort, wild yam,
catalpa, trumpet-creeper, centauria, mulleins, foxglove,
beardtongue, and many other fruits.
28. Large pods with small seeds to escape from small holes.--The large
ripe pod of the poppy stands erect on a stiff stem, with a number
of small openings near the top. The seeds are nearly spherical, and
escape, a few at a time, when the stem is shaken by the wind or some
animal, thus holding a reserve for a change of conditions. Here is
an illustration of ripe pods of a bellflower, _Campanula turbinata_,
nodding instead of erect.
[Illustration: FIG. 34.--Ripe pod of poppy on an erect, stiff stem,
ready for wind or animal to shake out a few seeds at the top.]
[Illustration: FIG. 35.--Ripe pods of bellflower bent over; the holes
opening when dry to allow seeds to be shaken out.]
The small holes are still uppermost, but to be uppermost in this case
it is necessary for them to be at the base of the pod.
29. Seeds kept dry by an umbrella growing over them.--When mature,
the apple of Peru, _Nicandra_, keeps every dry bursting fruit covered
with a hood, umbrella, or shed, so that seeds may be kept continually
dry and may be spread with every shake by the wind, or by an animal,
in rainy weather as well as in dry.
In the words of Dr. Gray, "The fruit is a globular dry berry, enclosed
by a five-parted, bladdery inflated calyx." The margins of the lobes
of the calyx curl upwards and outwards as the berry hangs with the
apex downward.
[Illustration: FIG. 36.--Mature fruit of apple of Peru covered by
an enlarged calyx.]
[Illustration: FIG. 37.--The same with the calyx removed.]
[Illustration: FIG. 38.--The same as Fig. 37, except that it is older
and somewhat changed.]
The berry is as large as one's thumb, and when ripe, bursts open
irregularly on the upper side as it hangs up under the calyx. As the
covering of the pod opens more and more, a few seeds at a time may
be rattled out by wind or animal. The numerous large and light fruits,
with calyx surrounding them, are each supported on a nodding stem,
stiff and elastic, which gives the wind a good chance to sway them
about. Water does not seem to get into the berries even when they
are torn open, for when it is poured over the branches it rolls off
the calyx roof as freely as from a duck's back. The fruits of
_Physalis_ are apparently kept dry in a manner similar to the apple
of Peru, although when first mature they are soft and juicy,
considerably like a ripe tomato.
30. Shot off by wind or animal.--The calyx of sage, bergamot, and
most other mints, remains dry and stiff, as a cup to hold one to four
little round nutlets as they ripen. The figure shows two of these
in section, as they are attached to the main stem of the plant, or
one of its branches. Observe the direction taken by the upper and
by the lower points of the calyx. When dry, the plant behaves somewhat
as follows: when the wind jostles the branches against each other,
or when an animal of some kind hits the plant, this movement causes
many of these cups to get caught; but the elastic stem comes suddenly
back to its place, and in so doing flips a nutlet or more from its
mouth one to six feet, somewhat as a boy would flip a pea with a
pea-shooter. In our garden, July 2, when plants of sage, _Salvia
interrupta_, were ripening their fruit, we found it difficult to
collect any seeds, but seedlings were observed in abundance on every
side of the plant, some to the distance of six feet. Plants dispersing
seeds in this manner have been called catapult fruits. Examine
ripening fruits of blue curls, pennyroyal, germander, balm,
horehound, dittany, hyssop, basil, marjoram, thyme, savory, catmint,
skullcap, self-heal, dragon's head, motherwort, and various dry
fruits of several chickweeds.
[Illustration: FIG. 39.--Ripe calyx of sage, first as pushed down;
second as let loose throwing nutlets.]
31. Seed-like fruits moved about by twisting awns.--Most of the
grains of grasses are invested with glumes, or chaff, and a
considerable per cent of the chaff has awns, some of which are well
developed and some poorly developed. The distribution of such grasses
depends on several agents--wind, water, and animals. The chaff and
awns of all are hygroscopic; that is, are changed by differences
caused by variation of moisture in the air. Sweet vernal grass, tall
oat grass, holy grass, redtop, animated or wild oats, blue-joint,
and porcupine grass are among them. When mature, the grain and glumes
drop off, or are pushed off, and go to the ground. When moist, these
awns untwist and straighten out, but when dry they coil up again;
with each change they seem to crawl about on the ground and work down
to low places or get into all sorts of cracks and crevices, where
the first rain is likely to cover them more or less with earth, after
which they are ready for growth.
32. Grains that bore into sheep or dogs or the sand.--Porcupine grass,
_Stipa spartea_, grows in dry soil in the northern states, but more
particularly on the dry prairies of the central portion of the United
States. This grass, when ripe, has a very bad reputation among
ranchmen for the annoyance the bearded grain causes them. The grains
are blown into the stubble among grasses with the bearded point down,
sticking into the soil. The first rain or heavy dew straightens out
the awns, which are twisted again as they dry. The bearded point works
a little farther with each change, and after twisting and untwisting
a number of times it gets down three or four inches into the sand,
often to moisture, where the awns decay and the grain germinates.
Here is an admirable scheme for moving about and for boring into the
ground. But this is not all. The grains are quick to catch fast to
clothing, as people move among the plants, and they are admirably
fitted for attaching themselves to dogs and sheep, which they annoy
very much. These animals transport the grains for long distances.
The twisting and untwisting of the awns enable the grain to bore
through the fleeces, and even to penetrate the skins and make wounds
which sometimes cause the death of the animal. Examine also seeds
of pin clover, Alfilerilla, which is becoming abundant in many parts
of the world.
[Illustration: FIG. 40.--Ripe fruit of pin clover, or Alfilerilla,
ready to twist into fleeces of sheep or into loose soil.]
33. Winged fruits and seeds fall with a whirl.--The large fruit of
the silver maple falls in summer. As these trees are most abundant
along the margins of streams, the fruit often drops into the water
and is carried down stream to some sand drift or into the mud, where
more sand is likely to cover them. Thus sown and planted and watered,
they soon grow and new trees spring up. But in many instances a strong
breeze, sometimes a whirlwind, has been seen to carry these mature
fruits from the tree to a distance of thirty rods.
[Illustration: FIG. 41.--Single fruit of silver maple.]
A thin sheet of paper descends more slowly than the same material
put in the form of a ball. On the same principle, many seeds and fruits
are flattened, apparently for a purpose; not that they may be easily
shot through the air by some elastic force, not to increase their
chances for attachment to animals, but to enable the wind to sustain
them the longer and carry them farther. Some seeds and dry fruits
are said to have wings, with the general understanding that they are
by this means better fitted to be sustained in air. We shall find
that all or nearly all flattened seeds and dry fruits, also winged
seeds and fruits, are one-sided, unbalanced, and more or less
twisted; consequently, in falling to the ground they whirl about,
and are thus kept much longer in the air than they would be if shaped
more like a winged arrow. Even the wings on the fruit of some of the
ashes are twisted, though many of them are flat. Experiments with
these things are sure to interest inquisitive children, or even older
persons, when once started right; they are likely to prove as
interesting as flying kites, skating, fishing, or coasting on the
hillside. Try experiments with seeds of catalpa, trumpet-creeper,
wild yam, pine, spruce, arbor vitae, and fruits of maple, box elder,
birch, hop tree, blue beech, ailanthus, ash, tulip tree,--in fact,
anything of this nature you can find, whether the name is familiar
or not. No two of them will behave in all respects alike.
[Illustration: FIG. 42. Winged seed of pine. Want of symmetry causes
it to whirl about while falling.]
34. Plants which preserve a portion of their seeds for an
emergency.--Many a great general or business man has learned by
experience and observation that it is usually unwise to exhaust all
resources in one effort. If possible, he always plans to have
something in reserve for an emergency--a loophole for escape from
difficulty. We have seen in many instances that plants are endowed
with the same trait. This is well illustrated by the way in which
the jack-pine, _Pinus_ [_Banksiana_] _divaricata_, holds in reserve
a portion of its seeds, to be used in case the parent trees are killed
by fire. In 1888 I made a study of this tree as it lives on the sandy
plains of Michigan. The tree is often killed by fire, and never sprouts
from the stump, as do oaks, willows, cherries, and most other trees.
The jack-pine grows readily and rapidly from seed dropped on the sand,
and begins to bear cones and seeds in abundance while it is yet only
a few years old, perhaps as young as five years in some instances.
The cones open slowly to liberate their seeds, some of them only after
months or even years, and in some cases they never open at all. I
have seen cones containing good seeds that had been nearly grown over
by the tree. Dry weather, the dryer and hotter the better, causes
many of these stubborn old cones to open their scales and allow the
seeds to escape. What can be the advantage in cones of this nature?
Let us see. A brisk fire passes over the ground at irregular intervals,
usually of from one to ten years; it licks up all dry leaves and sticks,
and kills the pine trees and all else above ground. The soil and the
trunks of trees are blackened, and by lack of reflection the heat
of the sun is rendered more intense; besides, the heat of the fire
acts slowly on the unburned cones as they are left on the dead trees.
By the time the quick hot fire has passed over, the cones have slowly
opened and begun scattering seeds on the vacant and newly burned
ground, at a time when there is the best possible chance for them
to grow. I picked a few unopened cones which, according to my judgment,
were from two to four years old. They were placed under glass in a
dark sheet-iron dish and exposed to the sun. The extra heat caused
the cones to open; many seeds were obtained and sown, and in five
days they began to come up, 95 per cent germinating. From the same
tree I selected at the same time older cones, which I believe to be
from four to six years old at least. From these, 225 seeds were sown,
191 of which germinated--about 85 per cent.
[Illustration: FIG. 43.--Cone of jack-pine closely covering its
seeds, often for several years.]
[Illustration: FIG. 44.--Cone of jack-pine as opened by heat, sowing
seeds.]