Second Article.
Our readers may remember a very simple experiment, but pregnant with
important results, which we described in our former article: namely,
if an onion plant, exposed to day light, be laid horizontally on the
ground, the extremities of the stem and roots will in the course of a
few hours turn themselves in their natural directions, the one upwards
and the other downwards; if a similar plant be placed in a dark cellar,
to which no light has access, the same things will take place; but that
which happens in a few hours in the one instance, will require as many
days in the other. From this experiment we were led to conclude that in
the production of the proper directions of stems and roots, two classes
of causes operate, namely, the light; and, secondly, some other principle
distinct from light. Our former article was devoted to the explanation
of the manner in which light causes stems to ascend, and roots to
descend; we shall now endeavour to investigate that other principle, less
powerful, but more universal, which shares in the production of the same
phenomena.
If the flower stalk of the common dandelion be split vertically into a
number of portions, each of these will be seen, spontaneously, to curl
outwards; the same tendency must be familiar to every one in celery
dressed for the table; if the root of the dandelion be split vertically
into two or more parts, these will likewise be found to curve, but in a
contrary direction from those of the stem; they will curl inwards. We
thus find that all the portions of stem placed round the central axis
have a natural tendency to curl outwards; while all the portions of root
round the central axis have a tendency to curl inwards. The stem may be
therefore considered as consisting of a number of springs placed round a
central axis, and all endeavouring to burst away from each other; while
the root, in a like manner, may be regarded as composed of a number of
springs placed round a central axis, and all pressing against each other.
These natural tendencies are overcome, in the living plant, by the mutual
cohesion of these parts or springs; but when this cohesion is removed by
the knife, their influence becomes acknowledged.
Now, if we imagine a number of springs, all of equal strength, and either
dragging away from each other or pressing together, it will be easily
understood that in such cases perfect equilibrium should result: for, of
two springs pulling in opposite directions, for either to overcome, it
is necessary that one should be the more powerful; and the same applies
to springs pressing against each other. As long, therefore, as a stem
consists of a number of equal springs, all endeavouring to burst away
from each other, its direction will be in a straight line; and as long
as a root is composed of equal springs pressing towards each other, its
direction, likewise, will be straight.
If a stem or root be placed for a certain length of time in a horizontal
position, the peculiar tendency to curvation of its parts will become
modified. If a stem which has been thus treated be split along its axis,
the part which, while it was in a horizontal position, was superior, will
have its tendency to curl outwards increased; while that which was under
the same circumstances inferior, will have its tendency to curl outwards
diminished. If a root be placed, during a certain period, horizontally,
and then split along its axis, the superior portion will be found to have
its tendency to curl inwards increased, while the inferior portion will
have the same tendency diminished. A horizontal position is therefore
found to increase the peculiar tendencies to curvation of the superior
parts of stems and roots, and to lessen those of the inferior half.
Now, we have already ascertained that while equal springs either pull
against or press towards each other, equilibrium is obtained; but if
from any cause the springs become unequal, the greater power may be
expected to overcome the less. When a stem or root has been kept for
some time horizontally, the upper half has its elastic power increased,
while the spring of the most depending portion has become diminished;
we have therefore now springs of unequal power placed round a central
axis, the superior being rendered more energetic, while their antagonists
have become weakened; it is reasonable, therefore, to expect that the
respective directions of roots and stems under such circumstances should
be obedient to the excess of elasticity which the upper half has acquired
over the lower; in other words, these stems and roots ought to direct
themselves in accordance with the natural tendencies of the superior
springs which enter into the structure of these organs. Now, the superior
springs of the stems have a natural tendency to curl outwards, or when
placed horizontally, upwards; and the superior half of the root has an
equally natural tendency to curl inwards, or, when placed horizontally,
downwards. Need we be surprised, therefore, if in obedience to these more
powerful springs the stems and roots of vegetables shall (as experience
shows us they will do) curve, after having been placed for some time in a
horizontal position, the former upward and the latter downward?
Let us now endeavour to explain the causes which produce these peculiar
and different tendencies to curvation of stems and roots, and for this
purpose it will be necessary for us to premise, that the fleshy substance
which constitutes the basis of vegetable structure is composed of a
multitude of little vesicles or cells, each perfectly distinct in itself,
and merely adhering by its external surface with those surrounding it,
while it contains a thick syrupy liquid; these cells, although pressed to
a certain extent against each other, are not so closely approached as to
obliterate completely the spaces existing between them, so that little
passages, called intercellular passages, continue to remain, during the
life of the plant, between the cells through which the ascending sap
rises in its passage to the buds. This ascending sap is not so viscid
a liquid as that contained in the cells: thus, the syrupy contents of
the latter must, according to the principle of endosmose described in
a previous article, absorb into the cells the ascending sap, in a way
similar to that whereby syrup placed in a bladder, immersed in a basin
of water, will attract the latter liquid through the membrane, until the
bladder be filled. While the sap continues to ascend, therefore, the
cells must necessarily continue swollen in proportion to their size.
If we examine the relative size of the cells in stems and roots, we
will meet with a remarkable phenomenon: in stems the largest cells are
situated towards the centre; but on the contrary, in roots, the largest
cells are placed near the circumference. Now, we have ascertained in
the preceding paragraph that all the cells have a tendency to swell in
proportion to their size: it follows that the central cells of stems
and the circumferential cells of roots possess the greatest tendency to
swell. The centre of a stem has therefore greater elasticity than the
circumference, while the circumference of a root has greater elasticity
than the centre. When this elasticity in either case is permitted to
exert itself by means of a vertical section, it causes each half of the
stem to curl outwards, and each half of the root to curl inwards. If the
influence of endosmose be acknowledged, the explanation is perfect.
But it may be said, what proof have we that endosmose operates in these
cases? An experiment instituted by Dutrochet, and repeated by the writer
of this article, sufficiently demonstrates its influence. A plant of
dandelion was immersed in syrup, and after a certain time the root and
stalk were severally split in a vertical direction: the tendencies to
incurvation of these organs were now seen to be completely changed from
what they are under ordinary circumstances; the parts of the stalk curled
inwards, those of the root outwards: this was exactly what might be
expected, if we suppose endosmose to be the cause of these phenomena;
placed in syrup, this thick fluid attracted liquid out of the cells,
which consequently shrunk in place of expanding; and the larger cells
contracting more than the smaller, the former elasticities became
reversed.
It remains to be seen why it is that when roots and stems are planted
for some time in a horizontal position, the proper elasticities of the
superior parts become increased, while those of the inferior become
diminished. These phenomena can be explained by recollecting that the
ascending sap is a heterogeneous fluid, composed of mucilage and syrup,
mixed with light water and carbonic acid, which have been drawn up
unchanged from the extremities of the roots, and are destined to escape,
or undergo decomposition in the leaves. It is not difficult to imagine
that this heterogeneous fluid contained in the intercellular passages
should be subjected to the influence of gravity; if this be admitted,
we can then understand how in a horizontal root or stem the heavier and
more viscid portion of the sap should descend into the inferior half,
and the lighter ascend between the cells of the superior half; endosmose
will take place in proportion to the difference of density between the
liquid in the intercellular passages and that contained in the cells;
therefore it will take place more energetically in the superior half,
where is the lighter fluid; and as the elasticity depends upon the
energy of endosmose, the upper portion will, according to its nature,
curve with greater force, while the elasticity of the lower part will be
lessened. This explanation acquires increased weight from the fact that
the specific gravity of the most depending portions of stems and roots
growing horizontally in the dark, is greater than that of the upper.
But we have stronger arguments in favour of the supposition that gravity
is essentially connected with the several directions of stems and roots.
These directions take place naturally in the “line of gravity,” that
is, parallel to a line drawn from the centre of the mass towards the
centre of the earth; at the same time it is to be remarked, that although
roots grow in the direction of gravity, that is, towards the centre of
the earth, stems grow in exactly the opposite way. An experiment made
by Mr Knight has been repeated by different philosophers, to determine
whether these directions of stems and roots bear to other physical laws
the same relation they do to gravity. Seeds permitted to germinate in
wet moss were attached to the circumference of a wheel made to revolve
constantly in a vertical manner; under these circumstances the roots grew
outwards, away from the circumference of the wheel, and the stems towards
its centre; the roots were thus found to obey the centrifugal force,
and the stems the centripetal; but while the wheel revolved vertically,
gravity and the centrifugal force were operating in the same direction.
It was necessary to cause them to act in different directions, and for
this purpose the wheel was made to revolve horizontally: in this case
the centrifugal force acted at right angles to the line of gravity, and
it was accordingly found, in obedience to the law of the composition of
forces, that the roots no longer grew towards the centre of the earth,
nor towards the circumference of the wheel, but in a plane between these
two forces; and the angle which they formed with the line of gravity
could be rendered more or less acute by increasing or diminishing the
velocity with which the wheel rotated. It was thus made evident that
roots and stems were influenced by physical laws, although growing in
opposite directions.
We have thus shown why roots grow downwards and stems towards the
heavens: in the dark these things arise through the influence of gravity
controlling endosmose, and thus producing the proper incurvations of the
parts of stems and roots. Under the influence of light the same phenomena
more energetically arise from the agency of this element over vegetable
growth.