SLEEP. 



679 



others fold in at the sides so as to embrace the 

 stem or to protect the flower which arises 

 from their axil. It is rare to see a movement 

 of the whole of a compound leaf, when its in- 

 dividual portions fold together ; such a move- 

 ment is seen, however, in the Mimosce. The 

 variety of positions assumed in sleep by the 

 subdivisions of compound leaves is very con- 

 siderable, and need not be here enumerated : 

 the phenomenon is best exhibited by the Lie- 

 guminoscs and the Qxalidece. 



Of the causes of this phenomenon, little 

 can be definitely stated. They are not to 

 be looked for solely in the operation of exter- 

 nal physical agents, such as light, heat, and 

 moisture ; for it can be easily shown that the 

 changes in question cannot be thus accounted 

 for, without attributing to the plants by which 

 it is exhibited a tendency to such periodical 

 manifestations inherent in their own consti- 

 tution. Thus, when sensitive plants are con- 

 fined in a dark room, their leaflets periodically 

 fold and open as usual ; the periods, however, 

 being somewhat lengthened. On the other 

 hand, when exposed to continued light, the 

 periodical folding and unfolding still occurs, 

 but the periods are shortened. And when 

 the plants are exposed to strong lamplight by 

 night, and excluded from all light by day, 

 their periods of sleep become extremely ir- 

 regular for a time, but in the end the plants 

 generally close their leaves during the day 

 and open them at night. No such modifica- 

 tions can be induced, however, in the Oxalidete ; 

 their periods of opening and closing their leaves 

 being unaltered by light, darkness, or by the 

 disturbance of the natural sequence of the two. 

 In the same manner it may be proved that 

 these movements cannot be laid to the account 

 of changes of temperature; for it appears from 

 the experiments of De Candolle, that they 

 continue to take place in plants exposed to 

 various degrees of temperature, as well as 

 in those left in air, provided that the heat or 

 cold be not sufficient to injure the health of 

 the plants. And by the same method of 

 exclusion, they can be shown not to be 

 dependent upon variations in the amount 

 of circumambient moisture ; since they con- 

 tinue equally well, cceteris paribus, when plants 

 are kept in stoves the humidity of whose at- 

 mosphere is uniform, and in some cases even 

 when the plants are entirely immersed in 

 water. We must conclude, then, that al- 

 though the exact time of the occurrence of 

 the phenomenon may be liable to modification 

 from the influence of external agents, its per- 

 formance is essentially independent of them, 

 aud must be referred to causes inherent in the 

 plant itself. 



The periodical closing of flowers is a change 

 which is obviously analogous to the sleep of 

 leaves. Many flowers only expand themselves 

 once, and speedily wither. Even in this case, 

 however, there is often considerable regularity 

 in the time of expansion, indicating periodi- 

 city. But in the flowers which remain fresh 

 for some days, some degree of alternation 

 between closure and expansion may be gene- 



rally discerned. There is no definite rela- 

 tion, however, between the sleep of flowers 

 and that of leaves ; for they may be united 

 in the same individuals, or be exhibited sepa- 

 rately in different species of the same genus. 

 Among other curious examples which show the 

 absence of connection between the two classes 

 of phenomena, is one cited by De Candolle 

 from Bertholletj the subject of it being an 

 Acacia cultivated in the garden at Orotava, 

 in which the leaves closed at sunset, but the 

 flowers then expanded, their numerous stamens 

 raising themselves up like tufts of feathers, 

 so as to become conspicuous ; whilst in the 

 morning, when the leaflets assumed their di- 

 urnal position, the filaments relaxed so that the 

 bunches of stamens gave to the flowers the 

 appearance of floss-silk, and the flowers them- 

 selves partly closed together. 



It has been ascertained by Meyen, that, by 

 the action of artificial light and darkness, the 

 usual hours for opening and closing may be 

 changed in flowers as well as in leaves. Thus 

 he found that after passing two days in a room 

 from which external light was excluded, but 

 which was lighted by four Argand lamps, 

 the flowers of Ipomcea purpurea, which natu- 

 rally open during the night, expanded in the 

 morning ; whilst those of Oxalis tetraphylla, at 

 the end of the fourth day of artificial illumi- 

 nation, opened in the evening, instead of at 

 their usual morning hour. 



Periodicity of Sleep. There can be little 

 doubt that a tendency to occasional repose 

 is inherent in the constitution of every 

 animal possessed of a sensorial apparatus; 

 and that this disposition is so arranged as 

 to correspond in its periodical recurrence 

 with the diurnal revolution of the earth. Al- 

 though we are accustomed to think that 

 " night is the time for sleep," and although, 

 in our own case and in that of most other 

 animals, darkness and silence favour repose, 

 yet it must be borne in mind that there 

 are many tribes of animals whose period of 

 activity is the very same with that during 

 which most others are wrapped in slumber. 

 Thus, among lepidopterous insects, we find 

 the activity of the greater part of the butter- 

 flies to be diurnal, that of the sphinges to be 

 crepuscular, and that of the moths to be 

 nocturnal. So among the insectivorous birds, 

 we find the diurnal swallow replaced during 

 the night by the goatsucker (or night-jar) ; 

 whilst the insectivorous bats are most active 

 during twilight. Among the raptorial birds, 

 again, we find the whole tribe of owls, with 

 only one or two exceptions, to be either noc- 

 turnal or crepuscular in their activity. And 

 among carnivorous animals we meet with a 

 similar diversity. As a general rule, the 

 vegetable-feeders of all tribes are diurnal in 

 their activity, taking their repose at night. 

 The nocturnal predaceous animals take their 

 repose during the day; and those whose 

 period of activity is the twilight, sleep partly 

 by night and partly by day. 



Notwithstanding this variety as to the 

 periods of sleep and activity, the complete 

 x x 4? 



