The use of poisonous fruits is an interesting subject for consideration. How is a plant benefited by producing them?
Mr. Grant Allen suggests with regard to a near relative of our Jack-in-the-pulpit that its brilliant scarlet berries are readily detected and eaten by birds; that such birds are consequently poisoned, and by decaying provide abundant nourishment for the germinating seeds. He adds that birds can not profit by experience and avoid the berries, as no bird ever lives to tell the tale.
At first this explanation seems very reasonable, and perhaps it is; but we have reason for doubting it, for we find that many fruits poisonous to mammals are eaten by birds without the slightest injury. The beautiful apple-like manchineel, which is most virulently poisonous, is eaten by tropical birds with the greatest impunity.
On the whole it seems very likely that some fruits are fatal to other animals but not to birds, and under all explanations poisons are doubtless a protection, at least, to the fruits which possess them.
Many fruits have been so highly cultivated by man that they can no longer set their seeds as originally. Our wild red cherry is a convenient morsel for even small birds; but its highly cultured relatives of the garden must submit their flesh to birds who can not eat stones as well. The case of the strawberry is different, however, for birds can scarcely take a morsel that does not contain numbers of the small, hard "straws," which are really the most essential parts of the plant, for each one incloses a seed.
In many cases Nature economically develops as little sweet pulp as will serve her purpose. In the wild red cherry, for instance, the stone occupies almost the entire fruit, there being only a thin layer of food substance. Often there is none whatever, and instead the fruit attains its ends by simulated attractiveness. The rosary bean temptingly displays its brilliant red seeds, which are in reality of stony hardness. Yet it does not wholly rely upon this artifice, for it is very probable that part of the seeds are scattered by the twisting dehiscence of the tough pod.
In some instances the deception is really wonderful. Some pods and seeds mimic insects so closely as probably to entice insectivorous birds to carry them, at least until the birds find out their mistake. It may be also that this appearance protects them from graminivorous birds. There are pods which curiously resemble worms and spiders and caterpillars. Our common castor-oil bean bears a superficial likeness to a beetle. Yet there are some most remarkable cases of mimicry where beetles are counterfeited in the minutest detail.
Fruits are also disseminated by mammals as well as birds.