Popular Science Monthly/Volume 21/August 1882/How Plants Resist Decay
THE destruction and decomposition of organic substances, both animal and vegetable, are promoted by the lower fungoids, particularly by yeast-plants and molds, which we may for brevity call rots. During life, that is, as long as a lively circulation is kept up, plants are protected against the attack of these ever-present organisms, but during the periods of rest, when the life-activity of the plant is reduced to a minimum, defense by the vegetative process is suspended. The older parts of plants, also, in which the normal circulation has become very limited, are poorly fitted to resist decay. It is, therefore, a question how it happens that perennial plants are so rarely attacked by the lower fungoids during their periods of rest.
The best protection is afforded by a firm epidermis, especially if it is fortified against the persistence of moisture upon it by a coating of wax. The importance of epidermal protection is exemplified in the North American opuntias, which bear the Central European winters quite well. If any part of the stalk has suffered an injury which has destroyed the outer skin, even if only for a very small space, decay begins at the wound, and spreads till it destroys the branch, unless warmer weather quickens growth in the weakened plant, when the process comes to a stop.
The most common protective armor of the higher and woody plants is the cork-bark. The corky substance is of itself extraordinarily tough, and even when dead is only very slowly destroyed by molds. Tree-barks also appear generally to contain chemical substances that operate as poisons upon the lower organisms. The most commonly diffused of such substances are tannin and other coloring-matters allied to it. Many barks also contain strong bitter matters and alkaloids, like salicin, pinipicrin, quercitrin, sesculin, chinin, aricin, strychnin, berberin, etc. The most persistent waxes also reside in the bark, and ethereal oils in individual cases, as in the laurels. The general diffusion of these substances in the bark is the more remarkable because they are rarely found in the wood or the annual leaves.
The subterranean parts of plants need the same protection as the stem. Swamp-plants in particular, which grow in a soil always undergoing decomposition, would fall a prey to decay very quickly without some especial defense. All plants whose organization does not insure them against the action of swamp-soil, perish alike, even under exposure in the winter, at any other time than during the growing season. The under-ground parts of plants growing in such soils are protected in part by the hard epidermis, partly by coloring-matter, as in the alder, comarum, and sanguisorba; or by bitter constituents, as in menyanthes; or by ethereal and aromatic substances, as in valerian and acorus; or by acrid matter, as in frangula and the Ranunculaceœ. Antiseptic substances, such as tannin, saponin, and phloridzin, are also found in the underground organs of plants that do not grow in swamps, and the strong essences of the rhizoma of ferns and of the punica-root belong to the same class.
Evergreen leaves, besides requiring means of defense against the lower organisms, need protection against the higher animals, which would consume them during the winter if they were quite accessible and enjoyable. Ulex and Ruscus, therefore, have thorny limbs, smilax and the evergreen brambles and roses have spines on their leaf-nerves, juniper and the holly-leaved plants have thorny leaves. The foliage of yew, arbor-vitæ, ledum, rhododendron, oleander, and laurocerasus is poisonous; and the palatableness of the leaves of pine and spruce, of laurel, ivy, and box, is at least very limited. Only the common underwood, consisting mostly of plants of the heath family, which are to a great extent covered during winter by leaves and snow, contain food for animals in their leaves and twigs. These leaves are likewise defended against rots by the poison in the poisonous kinds, by the hard, bright epidermis in the hollies, and presumably by chemical qualities; the heaths contain a coloring-matter.
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.
The great diversity of the chemical combinations that plants, instead of applying for their proper growth, store up in their tissues as a means of protection against the heat of the sun, the lower fungi, or animals, is really astonishing. Even in the liverworts we meet different substances neither the chemical nature nor the biological importance of which is clearly enough known. Among the ferns, only the under-ground stems are endowed with substances of strong qualities. Only single groups among the monocotyledonous plants, and of these the leaves (Araceæ) or the flowers (Melanthaceæ, lilies) of which show a higher organization, possess acrid poisons or alkaloids or aromatic substances. Among the conifers and dicotyledonous plants the strong substances are very widely diffused in the more enduring kinds.
We have not considered the coloring-matters and odors of the flowers and fruits in this sketch, for they do not serve for protection, but to attract animals.
It is worthy of remark that plants which produce particular substances may be naturalized in regions in which those substances are not required by them. The labiates and rues, which originally belonged to hot climates, still produce ethereal oils in Northern and Central Europe, although they no longer need protection against the hot sun. Closer reflection on the facts we have set forth will show that the real relation must be properly presented as a whole.
Numerous observations, nevertheless, are still necessary in individual cases to make the true significance of each particular phenomenon clear.—Kosmos.