864 MOVEMENTS OF PLANTS. 
rate mechanism, with a valve which closes directly an animal 
has entered, thus keeping it a prisoner (fig. 1177). It has 
been known for a long time to be capable of entrapping small 
invertebrates, but recently it has been found to catch young 
fish in the same manner. 
To plants which are thus stimulated to movement by 
chemical or mechanical means, the term irritable is applied ; thus 
Pres LEG: Hicetigie 
Fig. 1176. A sac of Utricularia, 
showing the external opening. 
Fig.1177. A vertical section 
of the same, showing the valve 
past which an entomostracous 
erustacean has entered, but 
cannot escape. 
it is by reason of their irritability that the leaves and stems of 
the Sensitive plants (fig. 373) droop on contact with any foreign 
body. 
(2) MovEMENTS INDEPENDENT, AT LEAST TO SOME EXTENT, OF 
EXTERNAL INFLUENCES.—-a. Periodical.—These movements are 
seen in some of the leaflets of certain tropical species of Desmo- 
dium, and more especially in those of Desmodiwm gyrans (fig. 
1178). The leaf in this plant is compound, and bears three 
leaflets; the terminal one, a, being much larger than the two 
lateral ones, b, b. There are also two other rudimentary leaflets, 
marked c, near the large terminal one. This large terminal 
leaflet, a, when exposed to the influence of a bright light, 
becomes more or less horizontal, but it falls downwards on’the 
approach of evening (fig. 1178, a). This movement is clearly 
analogous to the sleep of plants, and, therefore, comes under 
the head of movements depending on external influences, as 
previously described (page 861). But the lateral leaflets, b, b, 
exhibit a constant movement during the heat of the day, 
advancing by their margins towards the large terminal leaflet, 
and then retreating towards the base of the common petiole. 
This movement takes place first on one side and then on the 
other, so that the point of each leaflet describes a circle. The 
