Fig. 15.—The Dragon-tree of the villa Orotava.
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 Candolle[75] 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. Naegeli[77] 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] Beetroot[80] 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.