BOTANY. 
trees in leaf are nailed to a wall, and the po- 
sition of their leaves is consequently dis- 
turbed, they soon recover their natural di- 
rection. Light evidently acts as a whole- 
some stimulus to their upper surfaces, and 
as a hurtful one to the under. When the 
latter is forcibly presented for a long period 
to its rays, destruction is the consequence. 
Leaves seem to require occasional repose 
from the action of light on their upper sur- 
face ; for, when it is withdrawn from them, 
many leaves close or fold themselves toge- 
ther, as if in a state of relaxation, and spread 
themselves forth again at the returning 
beams of the morning. This is more espe- 
cially the case with winged leaves, as those 
of the pea kind. Those of the white aca- 
cia, robinia pseudo-acacia, have been re- 
marked by Bonnet to be over-excited by 
the sun of a very hot day, and to fold their 
upper sides together, in a manner directly 
contrary to their nocturnal posture. The 
effect of moisture upon leaves every one 
must have observed. By absorption from 
the atmosphere, they are refreshed, and by 
evaporation, especially when separated 
from their stalks, they soon fade and wi- 
ther. Aquatic vegetables, whose leaves are 
immersed in the water, both absorb and 
perspire with peculiar facility. Anatomical 
investigations have shewn that the nutri- 
tious juices, imbibed from the earth, and 
become sap, are carried by appropriate 
vessel's into the substance of the leaves. Mr. 
Knight, in his papers in the Philosophical 
Transactions, has demonstrated that these 
juices are returned from each leaf; not into 
the wood again, but into the bark. Hence 
a new and curious theory of vegetation has 
been established. It appears that the sap 
is carried into the leaves for the purpose of 
being acted upon by air and light, with the 
assistance of heat and moisture. By all 
these agents a most material change is 
wrought in its component parts and quali- 
ties, differing widely according to the di- 
versity of the species. Thus the resinous, 
oily, mucilaginous, saccharine, bitter, acid, 
or alkaline secretions are elaborated. The 
heedless observer of a leafis little aware of 
the wonderful operations constantly going 
on in its delicate substance, nor can the 
most enlightened philosopher explain more 
than a very small part of the chemical pro- 
cesses of which it is the immediate agent. It 
is scarcely necessary to observe how mate- 
rially plants differ in the flavour and quali- 
ties of their leaves, all which must .depend in 
a great measure on the operation of the 
leaf itself, for the common sap of plants, 
from which all their secretions are made 
differs very little in plants whose qualities 
are very unlike to each other ; those quali- 
ties depending upon the secreted fluids ela- 
borated principally by the leaves. 
The green colour of the organs in question 
is easily proved to be almost entirely owing 
to the action of light. Plants which grow in 
the dark are of a sickly white, which is the 
case with any parts artificially or acciden- 
tally covered with earth, as in cultivated 
cellery or asparagus, whose steals and leaf- 
stalks are purposely managed in this way to 
render their flavour and appearance more 
delicate. Such blanched parts soon be- 
come green on exposure to light. Leaves 
are subject to a sort of disease by which 
they become partially spotted or streaked 
with white or yellow. In this state they 
are termed variegated, and occasionally 
contribute to the ornament of our gardens. 
The whiteness frequently extends to the 
leaf-stalk, and sometimes to the branch, as 
may be seen in the variegated elder. Such 
varieties are propagated by cuttings, layers, 
or roots, but not by seed. They appear to 
be somewhat more tender than the plant in 
its natural state. One variety of the holly 
has, in addition to a yellow variegation, a 
beautiful tinge of purple, but this is a rare 
instance. In the amaianthus tricolor the 
leaves are naturally adorned with most beau- 
tiful and splendid colours, and in some 
other species of the same genus, with more 
uniform and less vivid tints. 
The irritable nature of some leaves is re- 
markable, not but that all leaves may truly 
be said to possess irritability with respect to 
light. The phenomena however to which 
we now allude are of the most striking kind. 
The sensitive plant, mimosapudica, common 
in hot-houses, when touched by any extra- 
neous body, folds up its leaves one after 
another, while their foot-stalks droop as if 
dying. After a while they recover them- 
selves again. Each leaf of the dionrea mu- 
scipula, or Venus’s fly-trap, is furnished 
with a pair of toothed lobes, which, when 
touched near the base, fold themselves toge- 
ther and imprison any insect that may be 
in their way. It is presumed that the air 
evolved by the body of the dead insect may 
be wholesome to the plant, for leaves are 
known to purify air impregnated with car- 
bonic acid gas, produced from the breathing 
of animals or the burning of a candle. The 
sarracenia, of which several species from 
America are now cultivated in our more 
