LATENT LIFE. - Latent life falls within the field of biological adaptations. It is to be understood as a reduction to a minimum of every vital activity, but one cannot speak of an interruption of it: interruption of life is death, and what is dead does not revive. Latent life takes place when environmental conditions become so unfavorable as not to allow a full vital activity.
The term latent life has been employed by biologists for conditions that are also quite diverse, whether physiological ones that intervene normally in the life cycle of organisms, such as the condition of seeds, of spores, of cysts, of diapause, of winter or summer lethargy (hibernation and estivation), or accidental conditions as in many cases of encystment and of hypnosis, or pathological ones, such as forms of thanatosis or of catalepsy. When latent life reaches its minimal expression it is indicated as "anabiosis"; for many authors the term latent life must be limited only to this aspect, considering lethargy on one side and hypnosis on another as quite different conditions, albeit with some point in common.
The important condition of all these aspects of latent life is the possibility of a return, by means of suitable processes, to active life. The first observation of the capacity for "return to life" of animals in "apparent death" is due to Antonie van Leeuwenhoek in 1701 on rotifers. Only forty years later an analogous observation was made on rotifers and on wheat eels by Baker and by Needham, who obtained the revival of these animals only by wetting the dried material. The sensation that this news aroused was such that these authors, under the weight of the harshest criticisms, had to disavow what they had observed. In 1776 Lazzaro Spallanzani, although he himself had at first attacked the conclusions of these authors, announced with the deposition of many experiments the truthfulness of these facts. But the polemics did not cease; indeed two currents of thought took shape, one of the "resurrectionists," headed by Doyère, and one of the "anti-resurrectionists," headed by Pouchet. The former even admitted a "momentary death" by desiccation. The polemic became so lively that a commission was set up in which men such as Berthelot, Broca and Brown-Séquard took part. The verdict was (1860) the acceptance of the resurrectionist point of view based on the fact that without water one cannot speak of life. But today no one any longer accepts this conclusion, inasmuch as desiccation is never complete and a minimum of life remains in the organism, and "reviviscence" is nothing but a re-exaltation of the quiescent vital processes. To Spallanzani is due the correct term of "minimal life," to Claude Bernard in 1879 that of latent life, which also well indicates this concept.
However, even after the authoritative affirmations on the concept of minimal life, some authors, such as Kochs (1890), returned to the concept of a true arrest of life; the term "anabiosis," introduced by Preyer (1891), serves to indicate this "suspension" of life. But these rekindlings of "resurrectionism" no longer find credit today. Let us examine some particular conditions of latent life, such as that of seeds and of spores. In this regard exaggerations have been uttered: for example, on the possibilities of germination of wheat found in the tombs of the Pharaohs; but Acton demonstrated that wheat after 30 years is no longer capable of germinating. The result obtained with the wheat of the Pharaohs was due to the naivety of the experimenters, victims of crude hoaxes. Nevertheless seeds present surprising resistance to unfavorable environmental conditions: De Candolle and Pictet obtained the sprouting of seeds brought to -200°C. Note that respiration is no longer appreciable already at -10°. It has been ascertained that refrigeration acts in the same way as desiccation. Becquerel (1904-10) attributed much value in these phenomena to the protective barriers that come to be formed and that impede the passage of air and of carbon dioxide. This author obtained a normal sprouting of fungal spores even after a stay of 25 months in a vacuum and of 24 days at a temperature between -180 and -253°C. It has been demonstrated that in quiescent seeds the various enzymatic systems persist.
Another form of latent life is that determined by low temperature even in higher organisms and which, in general terms, can be defined as "rigor." In a higher animal placed in the cold, respiration slows down, the internal body temperature decreases and the heart contractions slow down. These phenomena cause the animal to fall into a deep torpor and then into sleep; in animals in which this condition in the winter months is the norm, it is called "lethargy"; it is called "rigor" in those forms in which there is normally no capacity for lethargy. In these animals, if the lowering of temperature has been too great, the process is irreversible. One can nevertheless freeze cold-blooded vertebrates (fish and amphibians) and then have them resume their activity. Lethargy is a biological problem of great interest. The lowering of temperature is the stimulus that provokes it, but the animal is already predisposed to such a condition by its intimate anatomical and physiological constitution. The animal that falls into lethargy passes through several stages: first light sleep, then medium and then deep, and then becomes light again. The physiological activities gradually lower with a mechanism similar to that of anabiosis. The heartbeats are reduced and so is respiration.
Another condition of latent life is the encystment presented by many invertebrates. The formation of cysts has a different meaning: it can be an act of defense against the unfavorable conditions that come to be created in the environment, but
VITALE, abbot of SANKT PETER and bishop of SALZBURG, saint. Tomb slab in red marble of St. V., work of Master Hans (1497) - Salzburg, abbey church of Sankt Peter.
it can also be a preparatory phase of reproduction, as, for example, with sporulation. The cyst is formed with the elimination of water (which is a general phenomenon of latent life) and with the formation of protective integuments of various nature. The resistance of cysts is very remarkable: for example, the cysts of Artemia salina resume development from the dry state after about fifteen years. In them the enzymatic systems remain efficient (Urbani).
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Alberto Stefanelli