remarks Supplementary observations on aggregation in the roots of
plants.
I will here interrupt my account of the movements of the leaves, and
describe the phenomenon of aggregation, to which subject I have already
alluded. If the tentacles of a young, yet fully matured leaf, that has
never been excited or become inflected, be examined, the cells forming
the pedicels are seen to be filled with homogeneous, purple fluid. The
walls are lined by a layer of colourless, circulating protoplasm; but
this can be seen with much greater distinctness after the process of
aggregation has been partly effected than before. The purple fluid
which exudes from a crushed tentacle is somewhat coherent, and does not
mingle with the surrounding water; it contains much flocculent or
granular matter. But this matter may have been generated by the cells
having been crushed; some degree of aggregation having been thus almost
instantly caused. [page 39]
If a tentacle is examined some hours after the gland has been excited
by repeated touches, or by an inorganic or organic particle placed on
it, or by the absorption of certain fluids, it presents a wholly
changed appearance. The cells, instead of being filled with homogeneous
purple fluid, now contain variously shaped masses of purple matter,
suspended in a colourless or almost colourless fluid. The change is so
conspicuous that it is visible through a weak lens, and even sometimes
by the naked eye; the tentacles now have a mottled appearance, so that
one thus affected can be picked out with ease from all the others. The
same result follows if the glands on the disc are irritated in any
manner, so that the exterior tentacles become inflected; for their
contents will then be found in an aggregated condition, although their
glands have not as yet touched any object. But aggregation may occur
independently of inflection, as we shall presently see. By whatever
cause the process may have been excited, it commences within the
glands, and then travels down the tentacles. It can be observed much
more distinctly in the upper cells of the pedicels than within the
glands, as these are somewhat opaque. Shortly after the tentacles have
re-expanded, the little masses of protoplasm are all redissolved, and
the purple fluid within the cells becomes as homogeneous and
transparent as it was at first. The process of redissolution travels
upwards from the bases of the tentacles to the glands, and therefore in
34