Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer.

Date:
1875
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    of the pollen either to the stigma of the same flower (as in Berberis1) 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 decay2. 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 Megaclinium falcatum (an African orchid). The lateral leaflets of Desmodium gyrans3 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 220 C., from 2 to 5 minutes; the motion is often irregular, some- times interrupted, and then recommencing suddenly in jerks. The labellum of Megacli- nium falcatum4 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 different 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, i.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, Trifolium incarnatum and pratense, Phaseolus, various species of Oxalis, as 0. Acetosella, &c.5). 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 Megaclinium 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 1 [H. Muller (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 Kemer, Die Schutzmittel des Pollens gegen die Nachtheile vorzeitiger Dislocation und gegen die Nachtheile vorzeitiger Befeuchtung; Innsbruck, 1873.—Ed.] 3 For further illustrations see Meyen, Neues System der rflanzen-Physiologie, 1839, vol. Ill, p. 553- [The first account of Desmodium 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. 5 For further proof see Sachs, Flora, 1863, p. 468, where the literature of the subject is quoted. 3 E