CHAPTER XXII

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 bean 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 cultivation. One of them is known as wood sorrel. Fig. 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 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 compositous plants, allied to thistles, but the whole head, rather than the separate fruits, is transported. In some compositous fruits the pappus takes the form of hooks and spines, as in the “Spanish bayonets” and “pitchforks.” 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 fibers 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 woolen 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.]