350 



NATURE 



[August 23, 1877 



again to the primary petiole by another similar organ 

 from 2 to 3 mm. long and about I mm. thick. The base 

 of the petiole itself is transformed into a nearly cylindri- 

 cal contractile organ or " pulvinus," 4 to 5 mm. long and 

 2 to 2'5 mm. thick, furnished, like those of the secondary 

 petioles, with a number of long stiff hairs on the under 

 side, the upper side being only slightly hairy or entirely 

 glabrous. The pulvinus consists of a succulent paren- 

 chymatous tissue of the kind already described. The 

 cells of the under side are thin-walltd, those of the upper 

 side have walls about three times as thick. Each cell 

 contains a moderate quantity of protoplasm, a nucleus, 

 small grains of chlorophyll, starch, and, in addition, a 

 large globular drop consisting of a concentrated solution 

 of tannin surrounded by a pellicle. 



A somewhat slight concussion of the whole plant causes 

 the contractile organs of the primary petioles of all the 

 leaves to curve downwards, those of the secondary petioles 

 forwards, tho^e of the leallets forwards and upwards, 

 closing like the wings of a butterfly at rest. After irrita- 

 tion the pulvinus is flaccid, and more flexible than before. 

 A light touch on the hairs on the under side of the 

 pulvinus of the primary and secondary petioles is suffi- 

 cient to produce the movement ; in those of the leaflets 

 the lightest touch on the glabrous upper side. When 

 the temperature is high and the air very damp, the 

 irritability is much greater, and any local irritation incites 

 movements in the neighbouring organs, often in all the 

 leaves of a plant, a phenomenon which has been termed 

 " conduction of irritation. ' If one of the uppermost 

 leaflets is cut off by a pair of scissors, or its 

 pulvinus touched, or if it is placed in the focus of 

 a buining-glass, the irritation immediately takes place, 

 and this irritation is communicated to the ne.xt lower pair 

 of leaflets, and in succession to those at a greater distance ; 

 after a short time the leaflets of an adjoining secondary 

 petiole begin to fold together from below upwards, and the 

 same with the other secondary petioles ; finally, and 

 often after a considerable time, the primary petiole bends 

 downwards ; the phenomenon is then conducted to the 

 primary petiole of the ne.\t leaf below, as well as to the 

 next one above. It sometimes happens, however, that 

 particular parts appear to be less susceptible, and do not 

 display the phenomena in question until after they have 

 been once passed by. If the plant is left to itself, the 

 leaflets again expand, and the petioles reassume their 

 erect position after a few minutes ; the contractile organs 

 are then again irritable. 



That the phenomena of irritability are connected with 

 a displacement of water from the succulent tissue and its 

 replacement by air, is shown by the evident and imme- 

 diate change in colour ; the expulsion of the air from the 

 intercellular spaces and its replacement by water causes 

 the whole organ to assume a darker colour. If, more- 

 over, one of the large contractile organs is cut or 

 punctured, a drop of water immediately escapes from i', 

 and if placed in water it again absorbs it eagerly. A 

 variety of experiments by Sachs, Pfeffer, and Briicke also 

 appear to prove conclusively that the sensitiveness resides 

 in the under, and not in the upper side of the organ. 



With regard to external conditions which interfere with 

 the sensitiveness of the leaves of Mimosa, they become 

 rigid or insensitive from cold when, the conditions being 

 otherwise favourable, the temperature of the surrounding 

 air remains for some hours below 15° C. (59° F.) ; the 

 lower the temperature falls below this point, the more 

 quickly does the rigidity set in. With regard to the upper 

 limit, the leaves of the sensitive plant become rigid within 

 an hour in damp air of 40" C. (104° F.), within half an 

 hour in air of 45° C. (113^ F.), in a few minutes in air of 

 49"^ or 50° C. (122° F.). In water the rigidity from cold 

 sets in at a higher temperature, viz., in a quarter of an 

 hour between 16° and 17° C. (62° F.), and the rigidity 

 from heat at a lower temperature than in air, viz., in a 



quarter of an hour, between 36° and 40° C. A plant im- 

 mersed in water of from 19° to 2i°'5 C. remains sensitive 

 for eighteen hours or more. The maximum degree of 

 sensitiveness appears to be reached at 30° C. (86° C), at 

 which temperature the plant is so sensitive that the move- 

 ment is communicated to a number of leaflets almost 

 simultaneously. During the rigidity from heat, whether 

 in air or water, the leaflets are closed, as after irritation, 

 but the petiole is erect, and when irritated, turns down- 

 wards. 



If placed in the dark, the irritability to touch is not 

 at first affected, but disappears completely if the dark- 

 ness lasts for a day or more ; when again exposed to 

 light, the sensitiveness is restored after some hours. The 

 position of the parts is, however, very different from that 

 in the insensitive condition caused by heat ; the leaflets 

 remain quite expanded, but the secondary petioles are 

 directed downwards, and the primary petiole nearly hori- 

 zontal. The same effects are caused, though in a less 

 degree, when the supply of light is defective. M. Paul 

 Bert states that the irritability of the leaves of Mimosa 

 is destroyed by placing the plant under a bell-glass of 

 green glass almost as completely as if placed in the dark ; 

 the plants were entirely killed in twelve days under 

 blackened, in sixteen days under green gUss ; plants 

 placed beneath white, red, yellow, violet, and blue glasses 

 were still perfectly healthy and sensitive, though varying 

 in the rapidity of their growth. 



Drought also causes temporary rigidity. If a plant is 

 left unwatered for a considerable time, the sensitiveness 

 of the leaves perceptibly diminishes with the increasing 

 dryness, and an almost complete rigidity ensues, the 

 primary petiole assuming a horizontal position, and the 

 leaflets expanding ; watering the soil causes a return of 

 the sensitiveness after two or three hours. 



The same effect is produced if respiration is prevented 

 by exhausting the air. If a plant of Mimosa is placed 

 under the receiver of an air-pump and the air gradually 

 exhausted, the leaves first of all fold up, no doubt in con- 

 sequence of the concussion ; but the leaflets then expand, 

 the petiole becomes erect, and, while the leaves assume 

 the same position as after prolonged withdrawal of light, 

 they now remain rigid, resuming their sensitiveness when 

 again brought into the air. 



Finally, with regard to the effect of poisonous sub- 

 stances, J. B. Schnetzler has pointed out that the sub- 

 stances which destroy the contractility of animal sarcode 

 also destroy the irritability of the leaves of Mimosa and 

 other sensitive organs of plants. Curare has no preju- 

 dicial effect in either case, while nicotine, alcohol, and 

 mineral acids destroy both. The vapour of chloroform 

 causes transitory rigidity either in the expanded or in the 

 folded position resulting from irritation. 



The genus Mimosa is a very large one, forming, to- 

 gether with Acacia, the greater part of the sub-order Mi- 

 moseje of Leguminosie, and embracing about 200 species, 

 natives mostly of tropical America, extending aUo south 

 of the tropics, and into tropical Africa and the East Indies. 

 They have definite stamens (not more than twice the 

 number of petals), anthers not tipped by a gland, and a 

 pod, the valves of which, when ripe, arc either detached 

 entire or break into transverse joints. They are mostly 

 herbs, under-shrubs, or climbers ; a few erect much- 

 branched shrubs ; one or two trees : a large number are 

 spiny. It is only some of the species that are sensitive. 

 M. sciisili-'a. which is also grown in our greenhouses, 

 differs from M. piidica in the leaves having only two pairs 

 of pinnae, and each pinna only two pairs of ovate leaf- 

 lets, the inner leaflet of the lower pair being always very 

 small. M. albiila, another sensitive species occasionally 

 seen in hothouses, has elegant flower-heads of a pale pink 

 colour. Our illustration of M. pudica is taken, by per- 

 mission of Messrs. Longmans, from Thomd's " Textbook 

 of Botany," English edition. A. W. B. 



