_ON LEAVES. 99 
plants to the light during the night, and to the shade during the 
day,” says De Candolle, “I observed that at first the plants opened. 
_ and closed their leaves without any fixed rule, but after a few days 
they seemed to submit to their new position, and opened their 
leaves in the night, which was day to them, and closed them in 
the morning, which was their night. When exposed to a 
continuous light they had, as in their ordinary state, alternations 
of sleeping and waking ; but each of the periods were shorter than 
ordinary. On the other hand, when exposed to continued obscurity, 
they still presented the alternations of sleeping and waking, but 
very irregularly.” 
De Candolle adds that he was unable to modify the sleep of two 
species of Oxalis either by light or darkness, or by light at other 
than the natural periods. We may conclude with him, from these 
facts, that the movements of sleeping and waking are connected 
with some disposition inherent in the vegetable, but which is thrown 
into special activity by the stimulating action of light, which acts 
with different intensity on different vegetables, so that the same 
amount of light produces different results in different species. 
Having thus minutely studied the exterior character of leaves 
we shall endeavour to penetrate a little into their structure, and 
unveil the delicacy of their arrangements. 
A cellular tissue, to which we give in this case the name of 
parenchyma, from the soft, pulpy, and closely approximating cells 
of which it is composed, and which have been aptly compared to a 
mass of soap bubbles pressing against each other, fills all the 
interstices of the leaves left by the spreading xervures. It is 
covered, consolidated as it were, and protected against all external 
influence by the epidermis, a covering which spreads itself like a 
protecting mantle over the whole surface of the plant. Let us 
submit this parenchyma, nervure and epidermis, to microscopical 
examination, first considering the leaves of those vegetables which 
live on the air. 
If we pull to pieces with some care any leaf whatever, a frag- 
ment of transparent membrane without colour will be observed to 
detach itself from the leaf. If we place this moist shred upon a 
glass plate, and subject it to a magnifying glass, it will be found 
composed of large flattened cellules, having a contour, sometimes 
: H 2 
