460 PHYSIOLOGY CHAP. 



the mystics, in saints, and in men of genius generally, there is 

 an imperfect unification of the psychic personality, and more or 

 less manifest evidence of the existence of a secondary personality, 

 that is a subliminal ego. 



VI. The alternate sequence of waking and sleep bears some 

 resemblance at least in its first origin to the succession of day 

 and night. 



The rhythm of activity and rest, of functional energy and 

 torpor, of mobility and quiescence, of waking and sleep, can be 

 observed not only in man and in' most animals, but also in 

 plants, according to early observations of Clusius, Prosperus 

 Alpinus, and Linnaeus. In many plants, in fact, the leaves and 

 flowers expand during the day and shrink or fold up at night. 

 A number of botanists have studied this phenomenon experi- 

 mentally, and ascribe it to the diurnal and nocturnal variations 

 of meteorological, electrical, and hygrometrical conditions, to the 

 influence of light and heat, etc. They deny that the sleep of 

 plants is similar to that of animals, and reject the teleological 

 motive of protection from nocturnal cold and of rest, ascribed to 

 them by the earlier observers. In a recent monograph on the 

 Physiological Problem of Sleep (1913) Pieron maintains that 

 sleep -understood as a suspension of the sensori-motor activities 

 that bring the living being into relation with its environment 

 is not an absolute necessity. The so-called sleep of plants only 

 presents a superficial analogy with the slumber of the higher 

 animals. On the other hand, it is impossible to discover evidence 

 of sleep in many vertebrates. 



Still, in view of the incontestable biological analogy between 

 animal and plant protoplasm, the susceptibility of both to 

 anaesthetics, and particularly the existence of rudimentary sense- 

 organs in plants, it may legitimately be claimed that the 

 rhythmical oscillation of the functional activity of organs 

 which in the higher vertebrates takes the form of waking and 

 sleeping is a universal phenomenon, originally associated with the 

 physical changes of the atmospheric environment, and recurring 

 rhythmically with day and night. 



" The time of rest is night - time," writes Eousseau ; " it is 

 marked by Nature. It is a matter of constant observation that 

 sleep is quieter and more placid when the sun is below the 

 horizon." Daylight sleep, in fact, is less recuperative and less pro- 

 found and unbroken than night sleep. The depression of reflex 

 activity, slowing of the pulse and respiration, fall in thermogenesis, 

 etc., are less pronounced in sleep by day than by night (Vaschide, 

 1906). Darkness and silence are favourable to sleep by day, 

 which attains its maximum more slowly, and has an irregular 

 curve ; its onset is more sudden and its awakening more rapid ; 

 its dreams are more logical, its oniric current more reasonable, 



