VITA LATENTE
Latent life is to be understood within the field of biological adaptations. It is to be taken as a reduction to a minimum of every vital activity, but one cannot speak of an interruption of life, since death and what is dead do not return to life. Latent life comes about when environmental conditions become so unfavorable as not to allow full vital activity.
The term "latent life" has been employed by biologists for conditions that are often quite different, whether physiological ones that normally intervene in the life cycle of organisms, such as the condition of seeds, spores, cysts, diapause, winter or summer lethargy (hibernation and estivation), or accidental ones as in many cases of encystment and hypnosis, or pathological ones, such as forms of thanatosis or catalepsy. When latent life reaches its minimal expression it is indicated as "anabiosis"; for many authors the term "latent life" should be limited only to this aspect, considering lethargy on the one hand and hypnosis on the other as quite different conditions, even though they have some points in common.
The important condition common to all these aspects of latent life is the possibility of a return, through suitable processes, to active life. The first observation of the capacity for return to life of animals thought to be dead belongs to Antonio Leeuwenhoek in 1701 on rotifers. Only after forty years was a similar observation made on rotifers and wheat eelworms by Baker and Needham, who obtained the revivification of these animals only by moistening the dried specimens. The sensation caused by this news was such that these authors, under the weight of the most bitter criticism, had to disavow what they had observed. In 1776 Lazzaro Spallanzani, although at first he had attacked the conclusions of these authors, announced with the deposition of many experiments the truth of these facts. But the controversies did not cease and indeed two currents of thought emerged, one of the "resurrectionists," headed by Doyere, and one of the "anti-resurrectionists," headed by Pouchet. The former admitted outright a "momentary death" by desiccation. The controversy became so heated that a commission was set up in which men like Berthelot, Broca and Brown-Séquard participated. The verdict (1860) was the acceptance of the resurrectionist point of view based on the fact that without water one cannot speak of life. But today no one accepts this conclusion inasmuch as desiccation is never complete and a minimum of life remains in the organism, and "revivification" is nothing but a reawakening of quiescent vital processes. It is to Spallanzani that we owe the apt term "minimal life," and to Claude Bernard (1879) that of "latent life," which also well indicates this concept.
However, even after these authoritative statements on the concept of minimal life, some authors, such as Koch (1890), returned to the concept of a true arrest of life; the term "anabiosis," introduced by Preyer (1891), is meant to indicate this "suspension" of life. But these flare-ups of "resurrectionist" thought no longer find credence today. Some particular conditions of latent life are examined, such as that of seeds and spores. In this regard, some exaggerations have been made: for example, regarding the germination possibilities 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 gross frauds. Nevertheless, seeds show surprising resistance to unfavorable environmental conditions: De Candolle and Pictet obtained the germination of seeds subjected to –200° C! It should be noted 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 great importance in these phenomena to the protective barriers that form and that prevent the passage of air and carbon dioxide. This author obtained normal germination 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 dormant seeds the various enzyme 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, internal body temperature decreases, and heart contractions slow down. These phenomena cause the organism to fall into a deep torpor and thus into sleep; in animals in which this condition is normal during the winter months, it is called "hibernation"; it is called "rigor" in those forms in which there is normally no capacity for hibernation. In these animals, if the lowering of temperature has been too great, the process is irreversible. It is possible, however, to freeze cold-blooded vertebrates (fish and amphibians) and then have them resume their activity. Hibernation is a biological problem of great interest. The lowering of temperature is the stimulus that provokes it, but the organism is already predisposed to this condition by its intimate anatomical and physiological constitution. The organism that falls into hibernation passes through various stages: first light sleep, then medium and then deep, to then become light again. Physiological activities gradually decrease with a mechanism similar to that of anabiosis. Heartbeats and respiration are reduced.
Another condition of latent life is encystment presented by many invertebrates. The formation of cysts has a different meaning: it can be an act of defense against unfavorable conditions that arise in the environment, but