MOTILE AND IRRITABLE PARTS OF PLANTS. 785 



of the pollen either to the stigma of the same flower (as in Berberis^) or to those of 

 other flowers (as in Cynaraceae). The movements of petals caused by variations in 

 the light and temperature usually cause the flowers to open in the day, and therefore 

 render them accessible to the visits of insects for the purpose of pollination ; while 

 the closing of the flowers in the evening or in damp cold weather in the daytime, 

 — I. e. at times when insects would otherwise not visit them — protects the pollen- 

 grains from moisture and decay ^. We have no knowledge, on the other hand, of any 

 purpose in the economy of the plant served by the periodic and irritable move- 

 ments of foliage-leaves. 



A spontaneous periodic movement is seen most conspicuously in the few cases where 

 the period extends only over a few minutes, and the oscillation of the organ takes place 

 by day and night under a sufficiently high temperature, as in the small lateral leaflets of 

 the trifoliolate leaf of Desmodium gyrans (the Indian 'telegraph-plant'), and the labellum 

 of the flowers of MegacUnium falcatum (an African orchid). The lateral leaflets of 

 Desmodium gyrans^ are attached to the common petiole by slender petiolules 4 to 5 mm. 

 in length, the petiolules being the organs by the movements of which the leaflets are 

 carried round, their apices describing nearly a circle. One revolution takes, when the 

 temperature is above 22^ C, from 2 to 5 minutes; the motion is often irregular, some- 

 times interrupted, and then recommencing suddenly in jerks. The labellum of MegacU- 

 nium falcatum* narrows below into a claw traversed by three slender fibro-vascular 

 bundles, the curving of this portion imparting to the labellum a swinging motion up and 

 down. In a much larger number of other foliage-leaves endowed with periodic motion 

 the spontaneous periodicity is almost entirely concealed by the contractile parts being also 

 very sensitive to light, so that a cursory observation detects only the daily period, or the 

 diff"erent positions by day and night. If however these plants, or even cut branches 

 placed in water, remain for some days in the dark or in artificial light of unvarying in- 

 tensity, it is seen that the periodic movements do not cease, but continue even when 

 the temperature is constant, /. e. independently of any irritation resulting from change of 

 temperature. Under these circumstances the leaves are in a constant slow motion, indi- 

 cated by the varying positions at short intervals (as e.g. in Mimosa, Acacia lophantha, 

 TrifoUum incarnatum and pratense, Phaseolus, various species of Oxalis, as O. Acetosella^ 

 &c.^). A. P. de Candolle has also shown that the leaves of Mimosa make periodic 

 movements under a uniform artificial light. The behaviour of the lateral leaflets of 

 Desmodium gyrans and of the labellum of Megaclmium falcatum on the one hand, and that 

 of leaves which assume different positions by day and by night on the other hand, offer 

 a contrast in the following respect; in the former the internal periodic causes of the 

 movement are stronger than the irritation of the light to which they may happen to be 

 exposed, while in the latter these internal causes are outweighed by the irritation caused 

 by the varying amount of light under ordinary conditions. To this last category belong 



^ [H. Miiller (Befruchtung der Blumen durch Insekten ; Leipzig, 1873) has shown that the 

 irritability of the stamens of Berberis is a contrivance for cross-fertilisation rather than self- 

 fertilisation, — Ed.] 



2 [On contrivances for the protection of pollen from the influence of the weather, &c., see 

 Kerner, Die Schutzmittel des Pollens gegen die Nachtheile vorzeitiger Dislocation und gegen die 

 Nachtheile vorzeitiger Befeuchtung ; Innsbruck, 1873. — Ed.] 



^ For further illustrations see Meyen, Neues System der Pflanzen-Physiologie, 1S39, vol. Ill, 

 P- 553- [The first account of Besmodiiim gyrans, based on Lady Morison's observations, is by 

 Broussonet, Mem. Acad, de Paris, 1784, p. 616. — Ed.] 



* C. Morren, Ann. des sci. nat. 1843, 2nd series, vol. XIX, p. 91. 



^ For further proof see Sachs, Flora, 1863, p. 468, where the literature of the subject is quoted. 



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