MOTILE AND IRRITABLE PARTS OF PLANTS. 
883 
turns downwards while the leaflets turn forwards and upwards, till they partially cover 
one another in an imbricate manner. The folding-up of a leaf in the diurnal position 
when suddenly placed in darkness is the more energetic the longer the period of illu- 
mination has been, and inversely. If a plant which has been exposed to alternations of 
day and night be kept in darkness for a considerable time, the daily periodicity may 
continue for a time, according to Pfeffer, as a persistent effect, and then the more rapid 
spontaneous movements make their appearance. In cases in which they take place 
with considerable force {Oxalis, Trifolium) this persistent effect is not exhibited. In 
general the sensitiveness to the action of light is very different in different species. 
(3) Many leaves endowed with periodic motility or sensitiveness to light are also 
irritable to contact and concussion ^, as those of Oxalis Acetosella, stricta, corniculata, purpurea, 
carnosa, and Deppei ^, Robinia pseud-Acacia ^, various species of Mimosa, as sensitiua, pros- 
trata, casta, vinja, asperata, quadrivalvis, dormiens, pernambucina, pigra, humilis, and pellita, 
yEschinomene sensitiva, indica, and pumila, S^nithia sensiti'va, Desmanthus stolonifer, trique- 
trus, and lacustris. In the greater number of these plants a rather violent or often 
repeated concussion is requisite to set the parts in motion, which then always assume 
the position of sleep ; in other words, mechanical stimulation acts in the same way as a 
diminution of light. The sensitiveness is greater in Oxalis {Biophytum) sensiti'va and 
Mimosa pudica, where a very slight concussion or simple contact on the contractile 
organ suffices to cause immediate and considerable motion, which is due, when 
the plant is highly irritable, to conduction of the stimulus to the parts not touched. 
Although the position assumed in consequence of contact or concussion resembles the 
nocturnal position, yet it differs from it in its conditions ; the former is accompanied by 
flaccidity of the contractile organ, the latter by increased turgidity. 
Among irritable stamens may be enumerated the various species of Berberis^ i^^g' 
'Vulgaris, emarginata, cretica, and cristata), and of the sub-genus Mahonia. In contact 
with the corolla when at rest, they curve concavely inwards when the base of the inner 
side of the filament is lightly touched, so that the anther comes into contact with the 
stigma. 
There is a greater diversity in the phenomena produced by a slight blow or friction 
on any part of the filaments of various Cynaraceae (as Centaurea, Onopordon, Cnicus, 
Carduus, and Cynara) and Cichoriaceae (as Cichorium and Hieracium). The filaments 
which spring from the tube of the corolla bear the five firmly attached (not coherent) 
anthers, which together form a tube through which the style grows up while the pollen 
is escaping. At this time the filaments are irritable ; when at rest they are curved con- 
cavely outwards as far as the width of the corolla-tube will permit ; on contact or 
concussion they contract, become straight, and hence come into close contact along 
their whole length with the style which they enclose, lengthening again after some 
minutes and resuming their curved form. Since each separate filament is independently 
irritable, touching a single filament or a blow on one side only of the capitulum will irri- 
tate, according to circumstances, only one, two, or three of the filaments, and by the 
contraction of one side the whole of the reproductive organs will be bent to one side. 
By the displacement connected with this or the pressure of the other filaments on the 
corolla they are also irritated, and thus arises an irregular oscillating or twisting motion 
of the reproductive organs of the flower. If the whole capitulum is shaken, or if the 
hand is passed over the surface of the flower, or the flower is blown into, a ' creeping ' 
motion ensues of all the flowers in the capitulum. This phenomenon occurs only while 
the style is growing through the anther-tube and the pollen is being emptied into the 
tube ; the motion of the filaments effected by insects causes the anther-tube to be drawn 
* [Movements may also be caused by chemical stimuli; see Darwin, Insectivorous Plants, 1875.] 
2 From linger, Anatomie und Physiologie der Pflanzen, 1853, p. 417. 
3 Mohl, Flora, 1832, vol. II. No. 32, and Iiis Vermischte Schriften. 
* Goeppert, Linnsea, 1828, vol. III. p. 234 et seq. 
3 L 2 
