BOTANY. 



Vegetable 

 1'hysiolojry 



Causes af- 

 fecting ir- 

 ritability. 



-Turning 

 round of 

 !e<tves to 



without the least effect being produced on a single 

 leaflet. Extremely surprised at this circumstance 

 which I had then witnessed for the first time, I re- 

 quested another lady to make a similar attempt, and 

 immediately all the leaflets contracted." The expe- 

 riment was repeated by the first lady with the same 

 result as in the former trial. Probably this fact might 

 be resolved, or at least supposed to be resolved, by 

 its being referred to a difference in the electrical 

 states of the two individuals who made the expe'ri- 

 ments; but as the solution would be as inexplicable 

 as the fact itself, we have no hesitation in rejecting 

 it, and avowing our entire ignorance of the cause 

 which produces so extraordinary an effect. 



The causes inducing the contractions of different 

 plants, and of'the same plant, are very numerous. A 

 touch of the hand, a scratch, heat, cold, strong smells, 

 and many other stimuli produce this effect. By these 

 means many experiments have been made on plants 

 possessing irritability in perfection, and for that pur- 

 pose the Mimosa sensitiva has been generally select- 

 ed. Some stimuli seem, as we have hinted before, 

 destined to produce certain determinate effects in 

 certain plants, and are therefore applied by natural 

 means ; others again appear to have merely an acci- 

 dental influence. Thus, the Porliera hygromelrica, 

 a South American shrub, contracts its feathery leaves 

 on the approach of wet weather, which it indicates 

 with unerring certainty. The leaves of the Onoclea 

 sensibilis, a species of North American Filix, when 

 unfolding themselves, shrink on the least touch. The 

 Mimosa sensitiva, so frequently mentioned, is affected 

 by almost every means that can be applied. On the 

 other hand, the Dionwa muscipula does not close its 

 singularly constructed leaves until some animal has 

 entered into its bosom, nor does it open until all mo- 

 tion on 'the part of the confined insect has ceased. 

 Analogous facts have been observed connected with 

 the genus Drosera, which is found in considerable 

 quantity in this island. 



The most remarkable, however, of all those plants* 

 which appear not to be affected by any specific agent 

 for some definite event, is the Hedysarum gyrans, a 

 native of Bengal, which is cultivated in our hot- 

 houses. It has trifoliate leaves, the central one of 

 which is larger than the two others ; they are all at- 

 tached by articulations to a common foot-stalk. The 

 central leaflet remains iinmoveable in a horizontal 

 position during the day, and close to the foot-stalk 

 during the night ; the two smaller ones Unceasingly 

 describe an arc of a circle, the ascending of more ra- 

 pid than the descending motion. If the regular mo- 

 tion be interrupted, as soon as it recommences, it goes 

 on with increased vigour, and at length gradually 

 subsides into its more natural movements. Many 

 other examples of the same kind might be ad- 

 duced, if our limits permitted us to enlarge on 

 it. 



Light has a very powerful influence on vegetation, 

 the chief part of whic'h will be better discussed in 

 treating of it as a chemical agent. We may here re- 

 mark, that it also produces a very striking effect oh 

 the irritability of leaves ; plants being uniformly ob- 

 served to present their leaves to the side on which 

 the largest quantity of light is to be found. This 



fact is daily exemplified in hot-houses and on walls. Veg^ta'bl 

 The experiment can be easily made with a common Physiolog 

 geranium, or any of those plants that are usually " v~" 

 placed in windows; by reversing their usual position, 

 in a short time the leaves will be found to have turn- 

 ed completely round to the window. 



Some ingenious philosophers attempt to explain 

 the cause of this turning round of the leaves of plants 

 when placed in unnatural situations; and, in addition 

 to the influence of light, the agency of heat and 

 moisture have been called in, and very probably, in 

 some cases, they may act conjointly, though it is 

 highly improbable that this happens in every in- 

 stance. Bonnet supposed that the two last mention- 

 ed causes were the sole agents, and attempted to pro- 

 duce their effects by artificial means ; and he is said to 

 have succeeded in exciting motion in leaves by alter- 

 nately presenting a bar of red hot iron to the su- 

 perior surface, and a moistened sponge to the under 

 side. Whence he concluded that the former became 

 hardened by heat like parchment, and that the lat- 

 ter contracted with moisture. To give still greater 

 weight to this hypothesis, he constructed an artificial 

 leaf of parchment, of linen, and of brass wire, which, 

 on being exposed to heat and moisure, displayed the 

 same phenomena with the leaves of the Acacia. This 

 is all very pretty hypothesis, but there is little con- 

 fidence to be placed in it. It may be considered a 

 philosopher's waking dream. 



The lime necessary to restore any plant on which 

 any experiment has been made by touching, or any 

 other means, varies in proportion to its vigour, and 

 probably according to many other circumstances 

 which are yet unknown to us. 



Many plants close their leaves at a certain period Sleep, 

 of the day, and open them at another regular period : 

 this may be seen in any .garden at night j but it is 

 particularly remarkable in the Mimosa sensiliva, the 

 tamarind tree, and many others, but more especially 

 those of pinnated leguminous plants. This periodical 

 folding of the leaves has been called by LinnKUs the 

 sleep of plants, and the term is now generally used, 

 in spite of the objections that may be urged against 

 it. If we examine it accurately, there will be found 

 little or no reason for rejecting it on account of the 

 implied analogy ; for it is not at all improbable that 

 this folding of the leaves may, as Dr Smith expresses 

 it, " be useful to the vegetable constitution, as 

 real sleep is to the animal." This particular state 

 of the leaves also protects the flowers from the 

 dew. The phenomena of sleep were first noti- 

 ced by Garcias, in the tamarind tree, in his voy- 

 age to the Indies ; but the determination of the 

 general fact was reserved for the illustrious father of 

 scientific botany. The discovery was accidental, 

 though a less curious mind than that of Linnaeus 

 would have overlooked the circumstance which led 

 to it. Having reared a few plants of the Lotus, 

 the seeds of which he had received from Sau- 

 vages, he remarked in the morning a pair of very 

 thriving flowers, which disappeared at night. They, 

 however, reappeared the following day, which 

 excited Linnaeus' attention, and he commenced his 

 nightly examinations, by which he ascertained many 

 of the most valuable facts that we now possess. The 



