MOTION. 
♦he light penetrates. The new shoots of an 
espalier detach themselves from the wall 
which robs them of the air, the sun, and the 
light. It is ill quest of the same excellent 
gifts of nature, that the lateral branches of 
trees abandoning the direction of the stem, 
spread and extend themselves in a direction 
jiarallel to the soil, even when planted on a 
declivity. In like manner it appeal’s, that 
the roots penetrate more or less deeply into 
the ground, either in a perpendicular or 
horizontal direction, in proportion to their 
greater or less tendency to search for mois- 
ture. Thus it is a well known fact, that, in 
the neighbourhood of canals, ditches filled 
with water, and ground newly tilled, the 
roots of plants abandon their natural direc- 
tion, and as it were, steer their course to- 
wards the fine air, rich juices, and gratefid 
humidity, which their situation has placed 
within their reach. So strongly, indeed, 
are the roots of plants attracted by water, 
that they frequently relinquish the soil, and 
penetrate into the very heart of ditches and 
canals. This force of extension appears to 
be greater in roots than in stems. The 
branch surmounts an obstacle, by leaving 
its natural direction and over-topping it. 
The root, on the contrary, without once 
going out of its way, pierces the hardest 
soils, penetrates into walls, which it over- 
turns, and even into rocks, which it bursts. 
Although the natural motion of the trunk 
be to ascend, as was suggested above, yet 
is it forced oftentimes to descend ; for the 
trunk-roots growing out of some plants near 
the ground, and shrinking into it, serve, 
like so many ropes, to pluck the trunk an- 
nually lower and lower into the ground, 
along with them. If these trunk-roots break 
out only about the bottom of the trunk, 
then it gradually descends into the earth, 
and is converted into a root, but if the 
trunk is very slender, and the trunk-roots 
break forth all along it, then it creeps hori- 
zontally; the trunk-roots in question tether- 
ing it, as it trails along, to the ground, as in 
strawberry, cinquefoil, and mint. It may 
be ob.served, that the direction of the roots 
and stems of plants seems to be regulated, 
in a great measure, by the vapours which 
they contain, but more by those which arise 
from file soil in which they grow ; and, that 
heat, the sun, or the light, the causes already 
suggested, appear to contribute to that 
direction, only in so far as they augment or 
regulate the current of tliese nourishing 
vapours. 
Tnniks are not, however, the only parts 
of plants which direct their course towards 
the air and the light of the sun. There are 
flowers which, quitting their perpendicular 
direction, present their surface directly to 
that luminous body, and follow its situation 
in its diurnal course. This sort of motion 
has been called, by some \fliters, nutation ; 
and the plants which are subject to it, have 
been termed heliotropje; that is, turiyng 
with the sun. Of this kind are bastard- 
rocket, dyer’s-weed, sun-flower, turnsole, 
and the greatest part of the compound 
flowers with plain tongue-shaped petals. In 
these flowers, the disk or surface looks to- 
wards the east in the morning, the south at 
noon, and the west at night. The spikes or 
ears ot corn, which hang down by their 
weight, are observed, in like manner, to in- 
cline themselves towards the sun, never to 
the north. The stems of draba, tiientalis, 
and a species of bastard fever few with egg- 
shaped and notched leaves, incline or hang 
downwards during the night. 
The observations of Hales, and Bonnet, 
establish, that these motions are occasioned 
not by any twisting in the stem, but by the 
dryness of the fibres, which, by being ex- 
posed to the heat of the sun, contract, and 
thus determine the nutation of the flowers 
and young stems. It is in this manner that 
moisture and dryness alternately dilate and 
contract the plant improperly called the 
rose of Jericho ; au appearance which is 
likewise observed in the beards of oats, and 
in those of the capsules of crane’s bill. 
The direction of the leaves of several 
plants suffer considerable changes during the 
night. This is so certain, that if a botanist 
who is accustomed to the pai t or habit of 
plants, were to examine, in a summer night, 
the plants which cover any particular mea- 
dow, he would find several which he could 
not recognize by that character. The same 
changes happen when the moisture of the 
day corresponds to that of the night. 
The change of direction just mentioned 
is particularly sensible in compound leaves. 
During the heat of the sun in the day-time, 
the pinnated or winged leaves of several 
plants, particularly those of the pea-bloom 
or leguminous tribe, rise vertically upwards, 
and form a right angle with the common 
foot stalk ; the lobes, or lesser leaves, which 
stand opposite, being applied closely to- 
gether by their upper surface. Several 
simple leaves, particularly those of siges- 
becka, and Indian mallow, (ilrena) when 
their upper surface is exposed to an ardent 
sun, become, in like manner, concave ; 
which demonstrates their analogy with the 
winged leaves just mentioned. 
