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.
Fig. 249.—Explosion of the Balsam Pod.
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.
Fig. 251.—Winged Seeds of Catalpa.