DISPERSAL OF SEEDS
It is to the plant’s advantage to have its seeds distributed as
widely as possible. _It has a better chance of surviving in the
struggle for existence._ It gets away from competition. Many seeds
and fruits are of such character as to increase their chances of
wide dispersal. The commonest means of dissemination may be classed
under four heads: _explosive fruits_; _transportation by wind_;
_transportation by birds_; _burs_.
[Illustration: FIG. 249.—EXPLOSION OF THE BALSAM POD.]
[Illustration: FIG. 250.—EXPLOSIVE FRUITS OF OXALIS.
An exploding pod is shown at _c_. The dehiscence is shown at _b_.
The structure of the pod is seen at _a_.]
=Explosive Fruits.=—_Some pods open with explosive force and
discharge the seeds._ Even beans and everlasting peas do this.
More marked examples are the locust, witch hazel, garden balsam
(Fig. 249), wild jewel-weed or impatiens (touch-me-not), violet,
crane’s-bill or wild geranium, bull nettle, morning-glory, and the
oxalis (Fig. 250). The oxalis is common in several species in the
wild and in cultivation. One of them is known as wood sorrel. Figure
250 shows the common yellow oxalis. The pod opens loculicidally. The
elastic tissue suddenly contracts when dehiscence takes place, and
the seeds are thrown violently. The squirting cucumber is easily
grown in a garden (procure seeds of seedsmen), and the fruits
discharge the seeds with great force, throwing them many feet.
=Wind Travelers.=—Wind-transported seeds are of two general kinds:
those that are _provided with wings_, as the flat seeds of catalpa
(Fig. 251) and cone-bearing trees and the samaras of ash, elm, tulip
tree, ailanthus, and maple; and those which have _feathery buoys_ or
_parachutes_ to enable them to float in the air. Of the latter kind
are the fruits of many composites, in which the pappus is copious and
soft. Dandelion and thistle are examples. The silk of the milkweed
and probably the hairs on the cotton seed have a similar office, and
also the wool of the cat-tail. Recall the cottony seeds of the willow
and the poplar.
[Illustration: FIG. 251.—WINGED SEEDS OF CATALPA.]
=Dispersal by Birds.=—_Seeds of berries and of other small fleshy
fruits are carried far and wide by birds._ The pulp is digested,
but the seeds are not injured. Note how the cherries, raspberries,
blackberries, June-berries, and others spring up in the fence rows,
where the birds rest. Some berries and drupes persist far into
winter, when they supply food to cedar birds, robins, and the winter
birds. Red cedar is distributed by birds. Many of these pulpy fruits
are agreeable as human food, and some of them have been greatly
enlarged or “improved” by the arts of the cultivator. The seeds are
usually indigestible.
=Burs.=—Many seeds and fruits bear spines, hooks, and hairs, which
_adhere to the coats of animals and to clothing_. The burdock has
an involucre with hooked scales, containing the fruits inside. The
clotbur is also an involucre. Both are composite plants, allied to
thistles, but the whole head, rather than the separate fruits, is
transported. In some composite fruits the pappus takes the form of
hooks and spines, as in the “Spanish bayonets” and “pitch-forks.”
Fruits of various kinds are known as “stick tights,” as of the
agrimony and hound’s-tongue. Those who walk in the woods in late
summer and fall are aware that plants have means of disseminating
themselves (Fig. 252). If it is impossible to identify the burs which
one finds on clothing, the seeds may be planted and specimens of the
plant may then be grown.
[Illustration: FIG. 252.—STEALING A RIDE.]
SUGGESTIONS.—=174.= What advantage is it to the plant to have its
seeds widely dispersed? =175.= What are the leading ways in which
fruits and seeds are dispersed? =176.= Name some explosive fruits.
=177.= Describe wind travelers. =178.= What seeds are carried by
birds? =179.= Describe some bur with which you are familiar. =180.=
Are adhesive fruits usually dehiscent or indehiscent? =181.= Do
samaras grow on low or tall plants, as a rule? =182.= Are the
cotton fibres on the seed or on the fruit? =183.= Name the ways in
which the common weeds of your region are disseminated. =184.= This
lesson will suggest other ways in which seeds are transported.
Nuts are buried by squirrels for food; but if they are not eaten,
they may grow. The seeds of many plants are blown on the snow. The
old stalks of weeds, standing through the winter, may serve to
disseminate the plant. Seeds are carried by water down the streams
and along shores. About woollen mills strange plants often spring
up from seed brought in the fleeces. Sometimes the entire plant is
rolled for miles before the winds. Such plants are “tumbleweeds.”
Examples are Russian thistle, hair grass or tumblegrass (_Panicum
capillare_), cyclone plant (_Cycloloma platyphyllum_), and white
amaranth (_Amarantus albus_). About seaports strange plants are
often found, having been introduced in the earth that is used in
ships for ballast. These plants are usually known as “ballast
plants.” Most of them do not persist long. =185.= Plants are able
to spread themselves by means of the great numbers of seeds that
they produce. How many seeds may a given elm tree or apple tree or
raspberry bush produce?
[Illustration: FIG. 253.—THE FRUITS OF THE CAT-TAIL ARE LOOSENED BY
WIND AND WEATHER. ]