The Power of Movement in Plants | Page 4

stands, we know that
there is always movement in progress, and its amplitude, or direction,
or both, have only to be modified for the good of the plant in relation
with internal or external stimuli.
Besides describing the several modified forms of circumnutation, some
other subjects will be discussed. The two which have interested us most
are, firstly, the fact that with some seedling plants the uppermost part
alone is sensitive to light, and transmits an influence to the lower part,
causing it to bend. If therefore the upper part be wholly protected from
light, the lower part may be exposed for hours to it, and yet does not
become in the least bent, although this would have occurred quickly if
the upper part had been excited by light. Secondly, with the radicles of
seedlings, the tip is sensitive to various stimuli, especially to very slight
pressure, and when thus excited, transmits an influence to the upper
part, causing it to bend from the pressed side. On the other hand, if the
tip is subjected to the vapour of water proceeding from one side, the
upper part of the radicle bends towards this side. Again it is the tip, as
stated by Ciesielski, though denied by others, which is sensitive to the
attraction of gravity, and by transmission causes the adjoining parts of
the radicle to bend towards the centre of the earth. These several cases
of the effects of contact, other irritants, vapour, light, and the [page 5]
attraction of gravity being transmitted from the excited part for some
little distance along the organ in question, have an important bearing on
the theory of all such movements.
[Terminology.--A brief explanation of some terms which will be used,
must here be given. With seedlings, the stem which supports the
cotyledons (i.e. the organs which represent the first leaves) has been
called by many botanists the hypocotyledonous stem, but for brevity
sake we will speak of it merely as the hypocotyl: the stem immediately
above the cotyledons will be called the epicotyl or plumule. The radicle
can be distinguished from the hypocotyl only by the presence of
root-hairs and the nature of its covering. The meaning of the word
circumnutation has already been explained. Authors speak of positive
and negative heliotropism,*--that is, the bending of an organ to or from
the light; but it is much more convenient to confine the word

heliotropism to bending towards the light, and to designate as
apheliotropism bending from the light. There is another reason for this
change, for writers, as we have observed, occasionally drop the
adjectives positive and negative, and thus introduce confusion into their
discussions. Diaheliotropism may express a position more or less
transverse to the light and induced by it. In like manner positive
geotropism, or bending towards the centre of the earth, will be called
by us geotropism; apogeotropism will mean bending in opposition to
gravity or from the centre of the earth; and diageotropism, a position
more or less transverse to the radius of the earth. The words
heliotropism and geotropism properly mean the act of moving in
relation to the light or the earth; but in the same manner as gravitation,
though defined as "the act of tending to the centre," is often used to
express the cause of a body falling, so it will be found convenient
occasionally to employ heliotropism and geotropism, etc., as the cause
of the movements in question.
The term epinasty is now often used in Germany, and implies that the
upper surface of an organ grows more quickly than the
* The highly useful terms of Heliotropism and Geotropism were first
used by Dr. A. B. Frank: see his remarkable 'Beiträge zur
Pflanzenphysiologie,' 1868. [page 6] lower surface, and thus causes it
to bend downwards. Hyponasty is the reverse, and implies increased
growth along the lower surface, causing the part to bend upwards.*
Methods of Observation.--The movements, sometimes very small and
sometimes considerable in extent, of the various organs observed by us,
were traced in the manner which after many trials we found to be best,
and which must be described. Plants growing in pots were protected
wholly from the light, or had light admitted from above, or on one side
as the case might require, and were covered above by a large horizontal
sheet of glass, and with another vertical sheet on one side. A glass
filament, not thicker than a horsehair, and from a quarter to
three-quarters of an inch in length, was affixed to the part to be
observed by means of shellac dissolved in alcohol. The solution was
allowed to evaporate, until it became so thick that it set hard in two or

three seconds, and it never injured the tissues, even the tips of tender
radicles, to which it