Popular Science Monthly/Volume 20/March 1882/Longevity of Plants
|LONGEVITY OF PLANTS.|
THE extremes between which the duration of the lives of plants varies are widely removed from each other. On one side, we may see plants that begin and close their lives within a few hours or days; on the other side, plants, the lives of which may be estimated by the hundred or thousand years. The differences appear conspicuous, not only in the view of the whole vegetable kingdom, but also in single divisions of the same, and whether high or low forms are included. Inasmuch as these conditions are generally known and may be observed by every one who looks into the fields and woods, it may be of some interest to consider more closely the great diversities in the length of the life of plants. Such a consideration will afford a higher interest if we institute in connection with it a comparison of the manner in which the longevity of plants is related to their various systematic affinities; and further, if we include in our survey the question whether the different classes of longevities stand in immediate juxtaposition, or whether there are transitions between them. The results of the latter study will then lead us to the inquiry, by what causes the different periods of life are determined, and how they have been developed.
We begin with a comparison of the life-term and the manner of vegetation of the plant. The shortest periods are found in the cryptogams, among the lower algæ, the growth of which, taking place in an homogeneous and uniform element, water, renders any considerable differentiation of the single organs unnecessary, and thus makes it possible for the whole course of life to be gone through in much less time than can be done in the case of the more complexedly organized air living plants. In some of the simplest algæ the individual, consisting of a single cell, splits into two new cells which are, or quickly grow to be, like the mother, in form, at least; the life of the mother is ended by the formation of its two children, and these in their turn go on to another division and end their lives in the same manner. So short a period of life, lasting for only a few hours or days, is not known in any phanerogamous (or flowering) plant, not even among those which grow entirely in the water. The more complicated structure even of the simplest phanerogam requires at least several weeks before it can bring its fruit to a sufficient degree of maturity to cast its seed—the essential condition which the plant must fulfill before it can bring its life to a close without endangering the continuance of the species. In the short-lived cryptogams, propagation and the end of life come together. The same is the case with a large part of the phanerogams, while other plants, both cryptogams and phanerogams, do not close their lives with a single propagation of posterity, but repeat their fruit-bearings after definite periods of different lengths. Once-fruiting plants may bear seed either in the first or the second year and then die, or after a series of years, as in the case of the American aloe, while the several-times fruiting ones may bear either in the first or second year, or after several years, and then not die; and a number of degrees of transition occur between all of these life-methods. We turn now to a closer consideration of the causes of the differences.
In the simplest plants, consisting of a single cell, no differentiation of the parts can be perceived. Food is taken in on all sides alike, and is worked up in the whole plasma of the cell and applied to the growth of the cell-body, to the multiplication of cells by division, or is stored up as reserve food. This is possible because the cell either lives continuously in a uniform element, water, or, if it dries up, simply suspends its life while the dryness continues. The simplest structure, a uniform medium, the most speedy performance of the functions of life, and the shortest life-term, thus go together. Now, take a plant which no longer consists of a single cell, but is a structure of a larger or smaller number of different cells, which no longer lives in a uniform, constant element, but has its body partly in the ground and partly in the air, so that it is exposed to all the changes to which those elements are subject. In this case a complete differentiation of the organs for different life-functions, of which there is no need in the one-celled plant, becomes essential. There must be a root to take up fluid food; leaves to absorb gaseous matters and elaborate them under the influence of light into new substances to be applied to the growth and strengthening of the plant or to be stored up in particular parts as reserve food; and, besides, particular organs, distinct from the food-organs, for propagation, of the most complicated character, such as are not needed in water-plants, where cross-fertilization, so difficult to secure in the free air, is easily and directly effected. Furthermore, since plants rooted in the ground can not move about, the advantage which change of place offers to the extension of the species is compensated for by the seeds and fruits being endowed with peculiar arrangements by means of which they can spread their kind through a wider circle.
Evidently, all these organs can not be formed in a few days, but a considerable time is needed to bring their development and the fruiting process to perfection. Additional complications now enter into the life-relations of the plant. While the fruit-organs perish, when they are separated from the stem at maturity, the life of the plant itself does not necessarily cease when they are dropped, but has a possibility of continuance dependent on a variety of conditions, the chief of which is, whether the other organs have been exhausted or not.
The prolongation of life is also affected by a great number of outer conditions. The plant has to be subjected to the changes of the seasons; it has to struggle for support in competition with other plants; it has to deal with animals, some of which are beneficial, some damaging to it; and it may or may not find a sufficiency of food in the soil in which it is trying to grow. With respect to all its external relations, it must have the faculty of adapting itself, by extending its organs in one or another direction. And this adaptation is complicated by the fact that it has to be modified to meet different conditions, at different stages of development, and that the plant can flourish only when all its stages of growth are simultaneous with favoring outward circumstances, and can be completed while these continue.
These inter-relations of outward conditions and internal adaptiveness bring their advantages and their disadvantages. It is an advantage to annuals that they require only a short part of the year for their full development, and their existence is not endangered by winter frosts or the heats of tropical dry seasons; also that the abundance of seed, which the devotion of their whole life to one effort makes them capable of producing, enables them to spread very rapidly, and occupy all favorable situations in a few generations. It is, on the other hand, a disadvantage that they have to conquer their ground every year, and the tenderness of the structure of their parts above-ground renders them liable to destruction by animals while they have not vigor enough to send forth new shoots. Their continuance is also sometimes endangered by the occurrence of an exceptional season too short for them to mature their fruit.
The least advantageous habit is that of those plants that grow for several years, and produce fruit but once, and it is of the rarest occurrence. Exceedingly well off are tuberous and bulbous plants, whose under-ground perennial organs are protected from the influence of climate and the attacks of animals, have stored in them quantities of reserve food, and are fitted to send out new shoots as soon as favorable weather sets in, and to mature their fruit in a very short season. Hence we find such plants common where the seasons of growth, whether on account of the frigidity or the dryness of the climate, are shortest. They also seem to prevail where the climate is moderately moist and warm; and nearly all water phanerogams are of this class.
It is to the advantage of woody plants that they do not have to build up every year so much of the scaffold on which their flowers and fruits are to grow. If they have evergreen leaves, these contain, at the end of the season, a quantity of reserve food fit for assimilation, and are ready to go to work absorbing more immediately on the opening of a new season. They have also the title of possession in the struggle with other plants, and are protected against animals. They are, however, more exposed to injury from changes of climate than other plants.
Thus, each life-habit has its advantages and its disadvantages; and we may conclude from this that a change from a long-lived to a short lived form, or the reverse, may take place in consequence of adaptation to changes in outer circumstances. The origin of new habits depends, however, not on external circumstances alone, but upon a certain disposition in the plant, by means of which it is able to adapt itself to new conditions.
The climate has the most important influence upon vegetation, especially upon its endurance and its life-habits; for temperature, moisture, and light are the conditions on which the life of the plant depends in all its stages; and the movements of the air have also their effect upon its existence. When the climatic conditions change, the plant must accommodate itself to the change, and must thereby, directly or indirectly, suffer a modification in the length and kind of its life. The soil also, and changes in its condition, have their influence, and surrounding plants and animals have their effects, all of which the plant may feel and show very plainly.
Evidence that the causes which we have only briefly touched upon here play a part in determining the duration of the life of different plants is afforded by cultivation and by the geographical distribution of species. It is well known that when plants are taken into cultivation, and exposed to different temperatures, different degrees of moisture, and different soils from those they have been accustomed to, and direct attacks are made upon their growth, they often suffer changes of one kind or another, and the duration of their life especially is affected. Modifications of external influences may be brought about partly by sowing the seed at different seasons from the usual one, partly by artificial stimulation, pruning, manuring, withholding moisture, etc. The object in these processes has generally been to make the plant go through its course faster, so as to secure an earlier blooming or fruiting; and its life has been shortened without that being the real intention. The other object, of prolonging the life, is also desirable; and there are numerous examples in which the life of plants may be lengthened or shortened by different modes of cultivation.
Particularly interesting examples of the evolution of different life-terms are exhibited in the geographical distribution of plants. If we consider the whole earth as to its climate, we shall observe that in a few regions near the equator, that have a uniform climate, plants will grow all the year through without manifesting any periodical preferences. This is the case, for example, east of the Andes in Northern Brazil, in Guiana, and in Java, where the vegetation is green and blooms continuously, where most species become woody, and nearly all live long and bear fruit often; while the short-lived, once-fruiting species retire to the background. In other tropical regions, where a periodical climate is produced by differences in the moisture of the atmosphere, the long-lived plants prevail, and the ground is so occupied with them till the coming on of the dry season that the short-lived kinds can not find room upon it. The case is different in those regions where spots become barren of vegetation in consequence of the parching heat. Then, when the rainy season sets in, the annuals quickly spring up between the bulbous and tuberous herbs that are able to keep their places through the drought. The short-lived species are of most importance where a warm season alternates with a cold one, and the warm season lasts long enough for them to go within its term through the whole cycle of their life, from their seed-time to the ripening of their fruit. As the warm season becomes shorter the number of annuals is reduced, until finally, when the summer is not long enough for any of them to perfect their seed, they disappear altogether. Thus the persistent, often-fruiting species gain the monopoly on high mountains and in Arctic regions, but with the difference that in some districts they maintain themselves aboveground through the whole year without protection against the climate, while in others they exist through a long period of rest, protected against the injurious effects of the cold by means of their perennial parts under the soil or under the cover of an effective shelter.
Without attempting a closer review of these relations, or of the development of different life-terms in the successive periods of the development of the earth's surface and of the vegetable kingdom, we come to these comprehensive conclusions: A plant has to pass through two phases, one of vegetation, the other of propagation, both of which are conditioned upon the supply of food. In the simplest instances these phases are exhibited in one and the same cell, which, absorbing the food, elaborates it for its own growth, and gathers so much force that two new individuals are produced by division while the mother plant ceases, by this fact, to be. In another yet similar manner is accomplished the life of those plants which in their complicated structure present an extreme contrast to the simple cell. In this case also the whole feeding of the plant is performed with the sole object of adapting it to the purpose of propagation; but the end is reached in various ways and times, while the attainment of it is not always accompanied with the end of life. Some plants proceed in uninterrupted growth from their origin to the end of the propagation, collect forces on forces, and die exhausted as soon as propagation is accomplished. Others can not reach maturity by a short stage, but require a longer time for gathering their forces. This enforcement is gained either in an uninterrupted course, or, as is more frequently the case, by a series of alternate periods of growth and rest, followed by a period of vigor, when maturity of the fruit is at last reached and the plants die from exhaustion. A third group reach the stage of propagation after a shorter or longer period without giving up their whole strength to the formation of seed, but apply a portion of the nourishment they receive to the formation of perennial organs by the aid of which they continue to grow and become fruitful again, and competent to repeat the same processes an indefinite number of times.
Seeing different plants thus attain very different ages, we naturally inquire what are the causes of the diversity. They depend partly on the adaptation to external conditions induced by climate, soil, and environment, all of which, especially climate, determine the life-habits of plants in various ways, and seem also to affect the length of their life. But these external conditions can not of themselves force the plant to adapt itself to them; it must itself have a fitness to react upon influences acting upon it from without. This is really the case in various degrees, for the variability which a species exhibits by its individuals in different directions extends to the length and the manner of its life, and attests the faculty it has of suiting itself to those external conditions which are most favorable to its life-habits. We see also how the plants of the earth have undergone improvement in regard to the length of their life under the influence of external conditions and by means of their power of variation, and can conceive how development of this kind can and will go on; for a cessation of the process is not possible.
It is true that not only certain species but whole genera and whole families as well appear now to be firmly fixed as to their life-habits and the longevity of their individuals; such plants, in view of the certain mutability of external conditions must either become migratory, or, if they can not change, must eventually give way to better adapted kinds. Other species, on the contrary, are capable of improvement; their individuals vary among themselves as to the habit and duration of their life, easily adapt themselves to changing conditions, and are able to adopt other life-habits. These changes in life-habit are accompanied step by step by a morphological adaptation of the organs, and thus from species having a definite term of life may be developed new species with other terms of life. The formation of annual species appears especially to be making a progress which began when a periodical took the place of a uniform climate. These species are, on account of the abundance of their rapidly maturing fruit, capable of a rapid improvement, and have had their spread facilitated since the appearance of man, who by his methods of cultivation makes the ground fit for them in places where they otherwise would not thrive.
- Translated from "Das Ausland" for "The Popular Science Monthly."