THE MOVEMENTS OF PLANTS. 
199 
growth. The movements effected by growth on the alternate 
sides are confined to young growing leaves, whilst those 
effected by means of a pulvinus last for a long time. 
The evil effects which result if sleeping leaflets be prevented 
from pressing their upper surfaces together, so as to protect 
them from radiation, were well seen in experiments of Mr. 
Darwin’s, in which he pressed down the leaflets of Oxalis, 
Marsilia, &c., so that they could not bring their upper surfaces 
into contact ; the result was that the leaves were killed. Thus 
of twenty-four leaves of Marsilia extended horizontally, exposed 
to the zenith and to unobstructed radiation, twenty were killed 
and one injured, whilst a relatively very small proportion of 
the leaves, which had been allowed to go to sleep with their 
leaflets vertically dependent, were killed or injured. Mr. 
Darwin noticed that the difference in the amount of dew on 
the pinned open leaflets and on those which had gone to sleep, 
was generally conspicuous, the latter being sometimes absolutely 
dry, whilst the leaflets which had been horizontal were coated 
with large beads of dew. Another fact observable was that 
when leaves were kept motionless, they are more liable to 
injury than when they were slightly waved about by the wind, 
and thus got a little warmed by the surrounding air. 
Cotyledons, as well as leaves, may sleep ; the former seem 
to do so more commonly than leaves. Of 153 genera observed 
by Mr. Darwin, one or more species of twenty-six of these 
genera placed their cotyledons at night so as to stand vertically, 
having generally moved through an angle of at least 60°. In 
a large majority of genera the movement is a rising one. In 
all the species of Oxalis observed by Mr. Darwin, the cotyledons 
are provided with a pulvinus ; and he adds that this organ has 
become more or less rudimentary in 0. corniculata , in which the 
amount of upward movement of the cotyledons at night is very 
variable, but never enough to be called sleep. Similarly, in 
the Leguminosse, all the cotyledons which sleep have pulvini. 
As this organ has been referred to several times, it will be as 
well to describe it. It constitutes a cushion or joint, and 
consists of a mass of small cells, usually of a pale colour in 
the case of that attached to cotyledons, from the absence of 
chlorophyll, and having a convex outline. The development 
of a pulvinus follows from the growth of the cells over a small 
defined space of the petiole being almost arrested at an early 
stage. As a pulvinus is formed by this arrestment of the 
growth of its cells, movements dependent on their action may 
be long continued without any increase in length of the part 
thus provided; and such long-continued movements seem to 
be one chief end gained by the development of a pulvinus. 
