The Prolongation of Life

Part III — Investigations on Natural Death

I Natural Death Among Plants

Theory of the immortality of unicellular organisms—Examples of very old trees—Examples of short-lived plants—Prolongation of the life of some plants—Theory of the natural death of plants by exhaustion—Death of plants from auto-intoxication

It must surprise my readers to find how little science really knows about death. Although death has a preponderating place in religions, systems of philosophy, literature and folk-lore, scientific works pay little attention to it. This unfortunate fact explains, although it may not justify, the bitter attack made on science on the grounds that it is occupied with minutiæ and neglects the great problems of human life, such as death. When Tolstoi was absorbed by the problem and searched for some solution in the writings of scientific men, he found that the explanations were trivial or inexact. In consequence he was extremely indignant with the men who devoted themselves to the investigation of what seemed to him useless problems (such as the insect world, or the structure of cells and tissues) and who were yet unable to say what the destiny of man or death might be.

I am far from claiming to solve these problems; I can do little more than describe the actual state of the question of natural death. I hope in this way at least to prepare for scientific investigation, and to call attention to it as the most important problem of humanity.

By the use of the phrase “natural death” I mean to denote a phenomenon that is intrinsic in the nature of an organism and that is not the mere result of an external accident. Popular phraseology includes under natural death all cases due to diseases. But as such deaths can be avoided and are not due to qualities inherent in the organism, it is erroneous to include them in the category “natural death.”

In nature, death comes so frequently by accident that there is justification for asking if natural death really occurs. It used to be thought that death was the inevitable end of life and that the living principle contained within itself the germ of death. Accordingly, it was a surprising discovery that many low organisms die only by accident, and that if such accident be avoided, death does not fall on them. Unicellular organisms (such as infusoria, many other protozoa and low plants) multiply by simple division, the organism thus giving rise to two new organisms; the parent so to speak loses itself in its offspring without undergoing death. To criticisms of this mode of presentment of the facts, Weismann, who has attracted most attention to the view, replied as follows:—“In cultures of Infusoria, these little animals continually multiply by division and no dead bodies are found. The individual life is short, but it ends not in death but in transformation to two new individuals.”

Max Verworn,71 a physiologist of repute, objected that Weismann had overlooked the occurrence within the organism of a process of partial destruction, and that under certain conditions a complete organ of the infusorian body (the nucleus) dies and is absorbed. Such death of a part, however, is not followed by death of the whole, and as the continuous destruction of some of the cells in our own bodies is not regarded as our death, the criticism of the German physiologist cannot be accepted.

It is not only the extremely short-lived microscopic organisms that escape death. Some of the higher plants, which may attain to gigantic size, encounter death only by accidents. There is nothing to be found in the nature of their organisation which would seem to indicate that death is the inevitable or even probable result of their constitutions.

The longevity of some trees has long been notorious, as these appear to live for many centuries and to die only when they are overwhelmed by the ravages of a storm or killed by human agency.

When the Canary Islands were discovered, in the beginning of the fifteenth century, the early explorers were struck with the gigantic size of a dragon tree which was venerated by the natives as their tutelary deity. The tree stood in a Garden at Orotava in Teneriffe, and even in these early days, its huge trunk contained a gigantic hollow. The tree did not reward the worship of the natives, who were annihilated by the Spaniards, and it survived them for nearly four centuries. At the end of the eighteenth century it was seen by Humboldt,72 who found that the trunk was forty-five feet in circumference, and who attributed to it a great age because dragon trees grow extremely slowly. Early in the nineteenth century (1819) a furious tempest swept over Orotava and with a gigantic crash nearly a third of the crown of leaves and branches fell on the ground. Notwithstanding this shock, the monster survived for fifty years. Berthelot,73 who visited it in 1839, described it as follows:—“A dragon tree stood in front of my dwelling, grotesque in form, gigantic in size, which a storm had smitten without overwhelming. Ten men would have much ado to girdle its vast trunk, fifty feet in circumference at the ground. The huge column had a deep cave within it, hollowed by the ages; a rustic porch gave access to the interior, and the lofty dome, although half had been destroyed by a storm, still bore an enormous crown of branches.”

The famous dragon-tree got more and more damaged, and was finally overthrown by a storm in 1868. A few years after the catastrophe (in 1871) I myself saw the remains of the colossus, lying on the ground as a huge grey mass like some antediluvian monster. No accurate estimate of its age can be formed, but it must have lived several thousand years.

Trees have been known which were still older than the dragon-tree of Teneriffe. One of the best known is the baobab of Cape Verd, described by Adanson. “This remarkable tree was thirty feet in diameter when the famous French naturalist measured and described it. Three centuries earlier, some English sailors had cut an inscription on it, and Adanson laid this bare by removing three hundred layers of wood. On his observations Adanson based an estimate of 5,150 years as the age of the tree.74 The old cypresses of Mexico are thought to be still older. A. de Candolle75 concluded that the cypress of Montezuma was 2,000 years old when he saw it, and that the cypress at Oazaca was much older than the tree described by Adanson. In California, trees of the species Sequoia gigantea are three thousand years old, and Sargent, an American botanist, attributes to some of them an age of at least five thousand years.

The question of the nature of individuality in the vegetable world has been raised in connection with the longevity of trees. It has been asked if a tree is to be regarded as a single individual or as a colony of many plants like a branching polyp. It is a difficult question, but only of secondary importance from the point of view of this discussion. A. de Candolle,76 having paid special attention to the subject, came to the conclusion that trees do not die of old age, that, in the real sense of the phrase, there is no natural end of their existence. Many botanists agree with him. Naegeli77 holds that a tree several thousand years old dies only from external accidents.

It is plain that amongst the lower plants and the higher plants there are cases where natural death does not exist. Theoretically, life would have an unlimited duration, subject to the continuous replacement of the substance of the organism in the normal metabolism. It must not be inferred, however, that there is no such occurrence as natural death amongst plants. There are numerous cases where death comes quite apart from the agency of external forces. Even amongst closely related plants there are some cases where natural death does not occur, and others where it is normal. The lower fungi offer a good instance. Some of these pass through a longer or shorter vegetative stage and then the living mass breaks up into spores (Myxomycetes). The whole bulk of matter is not transformed, but the remnant consists only of cuticular secretions, not living cells. In other fungi, only some of the cells transform to spores, the others dying naturally.

One stage of the life history of some lower plants is of short duration. The prothalli of some cryptogams (Marsiliaceæ) live only a few hours, just long enough for the appearance of the sexual organs. When these are ripe the body of the prothallus and all its constituent cells fall a prey to natural death. In such cases there is a “corpse,” composed of dead cells and protoplasm. Even amongst the higher plants there are instances of an extremely short duration of life. Amaryllis lutea passes through all the stages of its life-history in ten days, the minimum time necessary for the sprouting of the leaves and flowers and the production of the seeds, after which it dies naturally.78 It is interesting to find that in the same family there are other plants notable for long duration of life. The Agave requires a century to produce its flowers before death comes naturally.

Everyone is familiar with the so-called “annual” plants which live only a few months, from the time when they sprout, until, after the production of seed, death comes to them naturally. The life of annuals, however, can be preserved for two or for several years. Rye is normally an annual, but some varieties are able to live for two years and produce two crops. The Cossacks of the Don have established this fact, and have cultivated a biennial variety of rye for many years.79 Beetroot80 is normally biennial, but has been changed to a plant which lives for from three to five years. Such instances are by no means unique.

Natural death can be postponed if the plant be prevented from seeding. Professor Hugo de Vries has prolonged the life of the Oenotheras he cultivates, by cutting the flowers before fertilisation. Under ordinary conditions the stem dies after producing from forty to fifty flowers, but, if cutting be practised, new flowers are produced until the winter cold intervenes. By cutting the stem sufficiently early, the plants are induced to develop new buds at the base, and these buds survive winter, and resume growth in the following spring.” (Extract from a letter of Prof. H. de Vries.)

The grass of lawns is usually mowed before it begins to flower, so as to prevent the ripening of the seeds and the death of the plant. When this is done, the grass remains continually green, and its life lasts for several years.

The connection between the seeding of plants and their natural death has been recognised for long, and is usually explained as being due to the exhaustion of the plant.

As I am not a botanist, and was anxious to know the views of botanists on natural death, I wrote to Prof. de Vries, as a universally accepted authority. The distinguished botanist replied to me as follows. “Your question is extremely difficult. I do not think that much is known as to the exact cause of the death of annual plants, but it is customary to attribute it to exhaustion.” All the botanists who have expressed opinions on this matter appear to hold a similar view. Hildebrand,81 the author of a memoir on the duration of life in plants, stated this view again and again. According to him “the life of annuals is usually short because they are exhausted by their extensive production of seeds (p. 116).” “Even amongst plants which produce seeds for several years, there are some which are prematurely exhausted by fructification and which die spontaneously” (p. 67). In the prothallus of many of the higher cryptogams, the formation of a single embryo is followed by natural death; as Goebel82 points out, the embryo completely absorbs the prothallus.

As plants generally obtain their food with ease, it is natural to ask what is the cause of the exhaustion after seeding. When a plant which cannot resist cold dies after it has produced its seeds in the end of the summer, the event is natural enough. But how can we explain the death of an annual plant which is growing in a rich soil, and which seeds in the beginning of the summer, as being due to exhaustion long before the winter cold. It frequently happens that after harvest new shoots spring up from grains which have fallen. The soil which can support this new vegetation cannot have been exhausted by the cereal in question; and there has been enough warmth for the new crop. It cannot be the external conditions which have caused the death of the parent plant. The explanation of this apparent contradiction has been sought in the constitution of the plant itself. Hildebrand remarks that “certain species have a constitution which tends to early fructification. As soon as the seeds have been set, the strength of the plant is exhausted in the swelling of the grains, so that the plant dies.” “Other species, on the contrary, are so constituted that they vegetate for a long time, before fruiting, after which, however, they also die. A third set of plants have such a constitution that “they do not die after seeding, that they can seed often and live for many years” (p. 113).

Being unable to indicate exactly the intrinsic mechanism of these different “constitutions,” several botanists explain them by a kind of teleological predestination. According to Hildebrand “the nutritive processes of a plant have no other purpose than to make it capable of reproduction; this final end, however, can be reached in different modes and after different periods of time” (p. 132). Goebel sets down similar views. “In heterosporous plants the whole course of the development of prothalli is predetermined. The prothalli, so far as we actually know, to use the phrase of theologians, are predestined; their fate is determined once for all” (p. 403). M. Massart83 expresses the same kind of view, when he says that “sometimes cells die because their work is finished, and they have no longer any reason for existing.”

Such an interpretation of the facts is quite opposed to determinism, and makes the problem of natural death in the plant world more difficult but more interesting.

The modern scientific conception of the universe excludes the idea of predestination. The relations between fructification and natural death must be regulated by the law of selection, according to which no organism survives if its reproduction is impossible. It occasionally happens that children are born without organs which are indispensable to life. Such monsters of different kinds being non-viable, cannot be said to be predestined to death, as they die because of defects in their structure. Others are born with all that is necessary for life, and survive for that reason, not because they are predestined to life. So also species of plants which develop incompletely and which die before they have produced spores or seeds, cannot survive; whilst those which die after having given birth to the next generation survive in their descendants. However quickly death follow the production of seed, the species will survive equally well. The cause of the natural death of plants must be sought, therefore, not in predestination, but in the mechanism of the organic processes.

Nothing seems more probable than that a plant should die when all its organic forces have been exhausted. It would be interesting, however, to ascertain the mechanism of that exhaustion, and this especially because it is often very difficult to imagine a cause for it. Many plants exist which produce several generations each season, in the same soil, without exhausting it. In perennial plants, some parts, such as the flowers, die periodically, although the plant itself is not exhausted. Everyone has seen that in geraniums some of the flowers wither whilst others are blooming, the process going on throughout the season. We can scarcely attribute such a natural death of the flowers to any exhaustion of the plant which continues to produce new flowers.

The fairly frequent prolongation of the life of plants is also out of harmony with the theory of natural death as the result of exhaustion. It sometimes happens that male plants produce female flowers abnormally; cases of this kind have been observed in willows, stinging-nettles, hops, and especially in maize.84 Here we have to deal with a kind of monstrosity, differing, however, from the non-viable monsters of the human race, in the respect that the production of female flowers on the male branches results in the prolongation of their lives. Generally the male branches die a natural death as soon as the pollen has been shed, and therefore some time before the death of the female flowers. If, however, a male branch bears a female flower which becomes fertilised, then the life of the branch is prolonged until the seeds ripen. If the natural death of the male flowers is the result of exhaustion due to the development of the pollen, how can we reconcile this with the prolongation of life in a case where the male branch has also female flowers to nourish and seeds to mature?

It is quite clear that natural death, in such cases, is the result of a mechanism more complex than simple exhaustion.

Prof. de Vries has already noted that the duration of life in plants depends on their vital processes. That view implies that there are some qualities inherent in its organisation which can prolong or shorten the life of a plant, and it is here that we ought to find the key to the problem of natural death in the vegetable world. However, to gain exact knowledge of such factors, it would be necessary to have information on many points in plant physiology which unfortunately are very imperfectly known. In this respect, the vital conditions of the simplest plants, such as yeasts and bacteria, have been investigated much more fully. It is true that such low organisms reproduce freely either by division or by budding, so that they are amongst the organisms in which natural death is not inevitable. None the less, in their lives phenomena occasionally present themselves which can be interpreted as cases of natural death.

At a time when it was still unknown that all fermentation was due to the action of microscopic plants, it had been observed that, in certain conditions, fermentation ceased much more quickly than in other conditions. For instance, when sugar is being transformed to lactic acid, it is useful to add chalk, as otherwise the fermentation stops before the greater part of the sugar has been acted upon. When, in 1857, Pasteur made his great discovery of the lactic acid microbe, he showed that that little organism, although it could produce lactic acid, was interfered with by an excess of the acid. To secure complete fermentation, it was necessary to neutralise the acid by the addition of chalk.

When the action of lactic acid is continued too long, it not only arrests the process of fermentation but definitely kills the microbe. It is for that reason that it has been found difficult to preserve the lactic acid ferment for a long time in a living condition. Amongst the ferments which have been isolated from Egyptian ‘leben’ by MM. Rist and Khoury85 there is one which is extremely delicate.

When it is inoculated deep in a nutritive medium, it dies in a few days, death, without doubt, being due to the lactic acid produced by the microbe from the sugar and not neutralised. As this transformation of sugar into lactic acid is a fundamental property of the microbe, depending on its constitution, the arrest of the fermentation and the death of the ferment in these definite conditions can be interpreted only as natural death due to auto-intoxication, that is to say to poisoning by a product of the physiological activity of the microbe itself. As death takes place at a time when the medium still contains enough sugar for the nutrition of the microbe, it is certain that it cannot be the result of exhaustion. This case of the lactic acid ferment is not unique. The microbe which produces butyric acid is also interfered with by the acid it secretes. M. G. Bertrand, who has examined carefully the microbe which produces fermentation in sorbose (sugar extracted from fruit of the service-tree) (Sorbus domestica) has informed me that this fermentation, too, ceases under the influence of the secretions of the microbes, and that the microbes undergo natural death at a time when the medium is far from exhausted of the nutritive material. The yeast which produces alcohol is also interfered with by an excess of alcohol, and as soon as a certain limit of alcoholic strength has been reached, fermentation stops. When the yeast is grown in media rich in nitrogen and poor in sugar, the plant takes the nitrogenous material and produces salts of ammonia. These alkalies damage the yeast and cause its death by auto-intoxication.86

In the examples that I have given, natural death was a result of the activity of the microbes, and was in correlation with their organisation. Such death can be avoided by changing the external conditions, and, if the acids or alkalies produced by these bacteria are neutralised, the bacteria survive. The facts are in harmony with those that I described in the case of the higher plants. By preventing the ripening of seed, the life of many annual plants may be preserved and the plants changed to biennials or perennials. In such cases death, although the result of the constitution of the plant, may be postponed.

We may ask then if the natural death of higher plants, usually attributed to exhaustion, cannot be explained more simply as the result of poisons produced in their metabolism. Many plants produce poisons which are fatal to animals and man. May they not also produce substances fatal to themselves? There is nothing improbable in the supposition that some of the poisons may develop when the seeds are ripening. By preventing the latter process, the ripening of the whole organism may also be prevented. Such a theory would explain the many cases of natural death which occur whilst the cell is far from having reached exhaustion. The equally numerous cases of partial death, such as that of flowers, whilst the same stem is still producing other flowers (e.g. geraniums) would be explained by a local action of the poisons not strong enough to kill the whole plant.

I must insist that this theory, that natural death of the higher plants, is the result of auto-intoxication, is a mere hypothesis which future investigations may disprove. If, however, it comes to be confirmed, it would explain the coincidence of death and fructification more simply than the hypothesis of predestination.

The higher plants may be subjects of auto-intoxication in the same fashion as bacteria and yeasts. If these poisons were produced before the ripening of the seeds, the plants would remain sterile, leaving no descendants, so that the race would become extinct. The production of poisons at the time of fructification would not interfere with the succession of generations, and the race would be preserved. As the poisoning is not necessary, it is easy to understand why many plants survive seeding and escape natural death. The Dragon-tree, baobab, and the cedars, which I spoke of earlier, would be examples of such escape.

Although the existence of auto-intoxication in the higher plants is still only a hypothesis, the natural death of bacteria and yeasts by poisons which they themselves produce is an ascertained fact.

In the plant world, therefore, there are examples of natural death (bacteria and yeasts) due to auto-intoxication, and there are other cases where high or low plants escape natural death.