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. 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.