We are naturally led to inquire what is the use of this movement which
lasts for so short a time? If very small objects, such as fibres of
meat, or moderately small objects, such as little flies or
cabbage-seeds, are placed close to the margin, they are either
completely or partially embraced by it. The glands of the overlapping
margin are thus brought into contact with such objects and pour forth
their secretion, afterwards absorbing the digested matter. But as the
incurvation lasts for so short a time, any such benefit can be of only
slight importance, yet perhaps greater than at first appears. The plant
lives in humid districts, and the insects which adhere to all parts of
the leaf are washed by every heavy shower of rain into the narrow
channel formed by the naturally incurved edges. For instance, my friend
in North Wales placed several insects on some leaves, and two days
afterwards (there having been heavy rain in the interval) found some of
them quite washed away, and many others safely tucked under the now
closely inflected margins, the glands of which all round the insects
were no doubt secreting. We can thus, also, understand how it is that
so many insects, and fragments of insects, are generally found lying
within the incurved margins of the leaves.
The incurvation of the margin, due to the presence of an exciting
object, must be serviceable in another [page 379] and probably more
important way. We have seen that when large bits of meat, or of sponge
soaked in the juice of meat, were placed on a leaf, the margin was not
able to embrace them, but, as it became incurved, pushed them very
slowly towards the middle of the leaf, to a distance from the outside
of fully .1 of an inch (2.54 mm.), that is, across between one-third
and one-fourth of the space between the edge and midrib. Any object,
such as a moderately sized insect, would thus be brought slowly into
contact with a far larger number of glands, inducing much more
secretion and absorption, than would otherwise have been the case. That
this would be highly serviceable to the plant, we may infer from the
fact that Drosera has acquired highly developed powers of movement,
merely for the sake of bringing all its glands into contact with
captured insects. So again, after a leaf of Dionaea has caught an
insect, the slow pressing together of the two lobes serves merely to
bring the glands on both sides into contact with it, causing also the
secretion charged with animal matter to spread by capillary attraction
over the whole surface. In the case of Pinguicula, as soon as an insect
has been pushed for some little distance towards the midrib, immediate
re-expansion would be beneficial, as the margins could not capture
fresh prey until they were unfolded. The service rendered by this
pushing action, as well as that from the marginal glands being brought
into contact for a short time with the upper surfaces of minute
captured insects, may perhaps account for the peculiar movements of the
Public-domain text, read in full here on John Shaqi.
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