882 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



sometimes interrupted, and then recommencing suddenly in jerks. The labellum of 

 Megarl'mium 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 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 intensity, it is seen that the periodic movements do not cease, but continue 

 when the temperature also is constant, i.e. independently of any irritation resulting 

 from change of temperature. Under these circumstances the leaves are in a constant 

 slow motion, indicated by the varying positions at short intervals (as e.g. in Mimosay 

 jicacia lophantha, Trifolium incarnatum and pratense, Phaseolus, various species of Oxalisy 

 as O. Acetosella, &c.^). After a certain time these movements cease. 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, so that 

 only the daily periodicity induced by the alternation of day and night is apparent. To this 

 last category belong the movements of the compound leaves of Leguminosae, of many 

 species of Oxalis, and of Marsilia. In the Leguminosae the common petiole is often 

 attached to the stem by a larger contractile organ or ^ pul'v'mus \' and in all the cases 

 just named the petiolule of each leaflet possesses a similar organ. If, as in the bipinnate 

 leaves of Mimosa^ there are secondary common petioles, these are also attached to the 

 primary petiole by contractile organs. These organs always consist of an axial fibro- 

 vascular bundle surrounded by a thick layer of turgid parenchyma. The other parts 

 of the leaves, the petiole as well as the lamina, are not spontaneously contractile, but 

 the alterations in their position are caused by the curvatures of the organs at their base. 

 The movement is either a curving upwards and downwards, as in Phaseolus, Trifolium, 

 Oxalis, and the common petioles of Mimosa, or is directed from behind and below in a 

 forward and upward direction, as in the leaflets of Mimosa. 



(2) Sensitiveness to variations in the intensity of light is exhibited with peculiar 

 distinctness in the leaves of Leguminosae and Oxalideae and of Marsilia, and the conse- 

 quent movements are effected by the organs which also produce the spontaneous periodic 

 movements^. These organs occur also in the leaves of many other plants, as Cannaceae 

 and Marattiaceae, but their irritability has not as yet been investigated. 



In the diurnal position produced by increasing intensity of light the leaves generally 

 have their surfaces completely unfolded and expanded flat ; in the nocturnal position 

 they are on the contrary folded up in different ways, being turned upwards, downwards, 

 or sideways. The leaflets of Lotus, Trifolium, Vicia, and Lathyrus are, for example, 

 folded upwards at night, those of Lupinus, Robinia, Glycyrrhiza, Glycine, Phaseolus, and 

 Oxalis downwards ; the common petiole of Mimosa turns downwards at night, that 

 of Phaseolus becomes erect ; the leaflets of Mimosa and Tamarindus indica^ turn laterally 

 forwards and upwards in the dark, those of Tephrosia caribcea backwards. When the 

 petiole and other parts of the same leaf are contractile, the curvatures of the various 

 motile parts may differ ; thus, for example, the petiole of Phaseolus turns upwards in the 

 evening, while the leaflets turn downwards ; the petiole of Mimosa on the other hand 



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



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



3 [See Somnus Plantarum, P. Bremer, Linn. Amoen. Acad. iv. p. 333.] 

 * See Meyen, Neues System der Pflanzen-Physiologie, vol. III. p. 476. 



