264 
PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
actually performs in vegetation, and in particular in the direction of stems, the im- 
portant part that has been attributed to it. 
3. On the Direction of Leaves. 
It is known to every one that the leaves of plants have a tendency to assume the 
same constant position with respect to their two surfaces. One of these surfaces is 
generally of a deeper green, smooth and glistening*, the nerves of the leaf being very 
little if at all prominent ; this is the surface naturally turned to the sky, and it is on 
that account called the upper, or superior surface. The other is of a paler green 
colour, full of little asperities or covered with short hair ; it has little or no varnish, 
and, as it is naturally turned towards the earth, it is called the under, or inferior 
surface. As far as regards the anatomical structure, M. Adolphe Brongniart has 
found out that the under surface of leaves contains a greater quantity of pneumatic 
cavities communicating with the external air by the apertures of stomata than the 
upper surface. It is to this accumulation of air that they owe, it seems, their whitish 
colour; and some leaves, like those of French beans, when placed in water under a 
vacuum, allow this air to disengage itself through the stomata, and, as it is replaced 
by water, the inferior surface of the leaf becomes of the same green colour as the 
upper surface. A long immersion in water without having recourse to a vacuum 
causes the air gradually to replace that liquid, and gives an uniform colour to the 
two surfaces of the leaf. In some leaves, in those of the Grasses for instance, there 
is hardly any difference between the two surfaces. In aquatic leaves, as in those of 
the Nymphcea, the air that exists in the pneumatic cavities appears, according to 
Dutrochet, to come in and out of them by the vessels of the footstalk. It seems 
that this air contains a little less oxygen gas than the atmosphere. Dutrochet 
makes out that the oxygen contained in the air of plants goes on diminishing from 
the leaves, where it is in the ratio of 16 per cent., to the roots, where it is only 8 per 
cent. His experiments have been made on Nymphcea lutea. 
The tendency of leaves to present their varnished surface to the sky and their 
pale surface to the earth, though very general, is not altogether without exceptions. 
Thus, the misletoe presents its leaves in all directions. The Riiscus aculeatus and 
some grasses turn their unvarnished surfaces upwards. It is the most deep-coloured 
surface that is directed towards the sky ; and if it were to the presence of air that 
they are indebted for their paleness, it is the surface most provided with pneumatic 
cavities that would be directed towards the earth. 
Among naturalists. Bonnet is the first who has examined the phenomenon of the 
direction of leaves. He has shown that if the position of a leaf be changed, so as to 
present to the sky its under surface, the leaf turns itself over spontaneously more or 
less quickly. This turning over is more rapid in young leaves than in old ones, 
* I have ascertained that this varnish is produced by a thin coat of wax, or rather myricine soluble in sul- 
phuric ether. 
