150 VEGETABLE PHYSIOLOGY. 



atmospliere, whether gloomy or bright, dry or moist, hot or cold, gives i-ise to 

 singular movements in leaves and flowers. Thus, during the night the leaflets of the 

 Bean and of Trefoils rise ; those of the Liquorice and of Hohinias hang vertically. 

 This phenomenon has been called the sleep of plants; and to prove that this sleeping 

 and waking depends on the absence and presence of light, plants have been, caused 

 to sleep at mid-day, by placing them in the dark ; whilst others have been wakened 

 at night by a strong artiflcial light. 



There are various exotic plants, which, waking by day and sleeping by night 

 in their native country, retain in our houses the habits of their climate, which 

 being the reverse of ours, they sleep during our day, and wake when the sun has 

 sunk below our horizon. Tropical plants wake and sleep with us as if we had a 

 perpetual equinox.' Certain plants exhibit movements induced by accidental external 

 stimuli; such is the Sensitive Plant {Mimosa pudica). Its periods for sleeping and 

 waking do not precise^ coincide with our night and day, its waking periods being 

 subject to vicissitudes depending on the slightest causes : a gentle shake, a breath 

 of wind, the passage of a storm-cloud, the falling of a shadow, offensive vapours, 

 the most delicate touch, cause the leaflets to droop suddenly, and closely overlap 

 each other along the petiole, which then droops also ; but soon after, if the cause 

 be removed, the plant recovers from this sort of faint, all its parts revive and resume 

 their first position. 



Venus' Fly-trap {Dionwa muscipula) is a small ISTorth American herb, whose 

 excitability is fatal to the insects which approach it ; its leaves terminate in two 

 rounded plates, joined by a hinge like the boards of a book, and fringed with 

 marginal bristles ; on their upper surface are two or three little glands which distil 

 a liquid attractive to insects';' when a fly touches these, the two plates close sharply 

 and seize the insect, whose efforts to escape increase the irritation of the plant, 

 which finally crushes it ; when the insect is dead and all movement has ceased, the 

 plates expand again, and await a fresh victim. These phenomena] which are"T!he' 

 effect of excitement, are not so exceptional as might be supposed ; many plants of 

 our climate offer analogous though much less remarkable examples. 



The opening of some fiowers is due to the stimulus of light : most open by day, 

 though some by night, as the Marvel of Peru (Mirahilis longifiora and Jalapa) ; others 

 open and close at various hours, and the hour of the day may be ascertained by 

 watching their habits. Linnaeus arranged his floral dock in accordance with these 

 periodical changes ; but such a clock, in our variable climate, is often too slow 

 or too fast ; it can only be correct in the torrid zone, where there are but few 

 atm.ospheric changes. 



The heat and moisture of the atmosphere also influence the daily motions of 

 flowers : certain species foretell rain by closing in the middle of the day, or by 

 remaining open in the evening, or by not opening in the morning. Attempts have 

 been made to construct a floral harometer from these observations, but its perform- 

 ances are far more irregular than those of the floral clock. 



• These statements are opposed to all the established phenomena of plant-life, as known to English 

 observers. — Ed. 



