882 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 
sometimes interrupted, and then recommencing suddenly in jerks. The labellum of 
Megaclinium 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, /. 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 Mimosa,, 
Acacia lophaniha, Trifolium incarnatum and pratenscy Phaseolus, various species of Oxalis^ 
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'vinus ; ' 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 y 
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 lamarindus indica^ turn laterally 
forwards and upwards in the dark, those of Tephrosia caribcBa 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 
^ C. Morren, Ann. des sei. 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. 
2 [See Somnus Plantarum, P. Bremer, Linn. Amoen, Acad. iv. p. 333 ] 
* See Meyen, Neues System der Pflanzen-Physiologie, vol. III. p. 476. 
