MOTILE AND IRRITABLE PARTS OF PLANTS. 
88l 
causes such a curvature of the contractile organs as to place the leaves in an ex- 
panded and completely unfolded position ; while any decrease in the intensity of the 
light produces the opposite curvature, causing the leaves to fold up. The expanded 
position is called that of waking or //le diurnal position, the opposite one that of sleep 
or the nocturnal position. In consequence of this sensitiveness to fluctuations in the 
light, these organs make periodic movements depending on the alternation of day and 
night, which, being induced by external causes, must be clearly distinguished from 
the automatic or those brought about by internal causes ; and the more so because 
both kinds usually occur in the same organ, and are combined in various ways^ 
In their sensitiveness to variations in the intensity of light these fully-developed 
organs resemble the growing organs referred to in Sect. 26. It has not yet 
been determined if any such parallelism exists with reference to variations of 
temperature. 
(3) In a smaller number of instances periodically motile foliage-leaves, as well 
as some reproductive organs which do not exhibit periodical movements, are irritable 
to touch or concussion. If a particular spot of the organ is only lightly touched or 
subjected to a slight rubbing from a solid body, the side which is touched becomes 
concave or contracts. The same effect is produced if a stronger impulse acts on 
any other part of the irritable organ, which then excites the irritable part. If the 
motile part has curved in consequence of the mechanical irritation, it afterwards re- 
sumes its previous position, and is then again irritable. 
The biological significance of these various forms of movement in the economy 
of the plant is known only in a few instances, as in the case of irritable stamens, 
where the insects that visit the flowers cause the irritation and consequent alteration 
in the position of the stamens, these movements being serviceable for the conveyance 
of the pollen either to the stigma of the same flower (as in Berberis'^) or to those of 
other flowers (as in Cynaracese). 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 ^ 
(i) The spontaneous periodic movement is seen most conspicuously in the few cases in 
which 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 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, 
^ This distinction, partly founded on facts that have long been known, is very necessary for 
a clear insight into the phenomena, and was first brought forward by me in the treatise on the 
various immobile conditions of the periodically motile and irritable parts of plants ('Flora,' 1863). 
2 [H. Müller (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 for self- 
fertilisation.] 
^ [See note 2 on page 873.] 
* For further illustrations see Meyen, Neues System der Pflanzen-Physiologie, 1839, '^ol- ^l^^- 
P- 553- [The first account of Desmodium gyrans, based on Lady Morison's observations, is by 
Broussonet, Mem. Acad, de Paris, 1784, p. 616.] 
3 L 
