Among the fruits, the juicy ones are designed to be eaten by animals, which are to serve as the medium for scattering their seeds. It is, therefore, of advantage to the plant to have its fruit valuable to some animal. The fall season affords an excessive abundance of fruits, but the best and most palatable ones are exposed to speedy destruction. It is, therefore, an advantage to animals, especially to birds, and to the plants likewise, if a few fruits have keeping qualities, that is, are able to resist decay, even if it be partly at the expense of their pleasant taste. This is the case, for example, with the berries of juniper, yew, holly, viburnum, and cowberry, whose persistence appears to depend partly on a hard epidermis, partly on chemical qualities; on an ethereal oil in the juniper-berry, apparently on benzoic acid in the cowberry. Ivy-berries do not ripen till winter.
Seeds are protected by their hard casings or by chemical substances. The poison contained in seeds may in part answer the purpose of preventing their being consumed by animals; but many seeds, as for instance the aromatic seeds of the umbelliferæ and other plants, contain not poisonous but antiseptic substances. The fatty oil, which is so abundantly present in seeds, is perhaps as valuable as a means of protection as for food. The oil as well as the shell of the seed prevents the entrance of water at low temperatures; and, unless water is present, the dry seed can not be attacked by the germs of decay.
If we survey the vegetable products that afford active chemical agents, we shall find that they are predominantly the bark, roots, and seeds. The coloring-matters, the bitter products, the alkaloids, and the poisonous substances are for the most part obtained from these organs. The leaves which afford such powerful matters are generally evergreen. Indeed, there are poisonous plants (among the nightshades, Araceœ, and Personatœ) and certain poisonous bushes (dogbanes and cashews) which are protected by this quality against the teeth of animals. The ethereal oils serve further in many plants, as among the labiates, the rues, the myrtles, and some geraniums, for protection against the heat of the sun, the reduction of temperature produced by the evaporation of the oils compensating in some degree for the insufficiency of the water which the plants are able to draw from the soil. Aside from these particular cases, however, chemically differentiated substances do not occur abundantly in the leaves or the wood of summer-green plants. Yet a protection of the wood against the germs of decay is evidently not at all superfluous. Wounds are often made upon trees by mechanical injuries, dying limbs, etc., from which decay may penetrate to the interior of the tree. Hence, we quite often find the wood rotten in the interior of a living tree. It is, therefore, also an advantage if the wood is defended against the attacks of rot by its finer texture, by gums, or by antiseptic substances like camphor, quassine, berberin, or columbin.
