Insectivorous Plants | Page 4

is necessary, in the first place, to describe briefly the plant. It bears

from two or three to five or six leaves, generally extended more or less
horizontally, but sometimes standing vertically upwards. The shape and
general appearance of a leaf is shown, as seen from above, in fig. 1, and
as seen laterally, in fig. 2. The leaves are commonly a little broader
than long,
FIG. 2. (Drosera rotundifolia.) Old leaf viewed laterally; enlarged about
five times.
but this was not the case in the one here figured. The whole upper
surface is covered with gland-bearing filaments, or tentacles, as I shall
call them, from their manner of acting. The glands were counted on
thirty-one leaves, but many of these were of unusually large size, and
the average number was 192; the greatest number being 260, and the
least 130. The glands are each surrounded by large drops of extremely
viscid secretion, which, glittering in the sun, have given rise to the
plant's poetical name of the sun-dew.
[The tentacles on the central part of the leaf or disc are short and stand
upright, and their pedicels are green. Towards the margin they become
longer and longer and more inclined [page 5] outwards, with their
pedicels of a purple colour. Those on the extreme margin project in the
same plane with the leaf, or more commonly (see fig. 2) are
considerably reflexed. A few tentacles spring from the base of the
footstalk or petiole, and these are the longest of all, being sometimes
nearly 1/4 of an inch in length. On a leaf bearing altogether 252
tentacles, the short ones on the disc, having green pedicels, were in
number to the longer submarginal and marginal tentacles, having
purple pedicels, as nine to sixteen.
A tentacle consists of a thin, straight, hair-like pedicel, carrying a gland
on the summit. The pedicel is somewhat flattened, and is formed of
several rows of elongated cells, filled with purple fluid or granular
matter.* There is, however, a narrow zone close beneath the glands of
the longer tentacles, and a broader zone near their bases, of a green tint.
Spiral vessels, accompanied by simple vascular tissue, branch off from
the vascular bundles in the blade of the leaf, and run up all the tentacles
into the glands.

Several eminent physiologists have discussed the homological nature of
these appendages or tentacles, that is, whether they ought to be
considered as hairs (trichomes) or prolongations of the leaf. Nitschke
has shown that they include all the elements proper to the blade of a
leaf; and the fact of their including vascular tissue was formerly
thought to prove that they were prolongations of the leaf, but it is now
known that vessels sometimes enter true hairs. The power of movement
which they possess is a strong argument against their being viewed as
hairs. The conclusion which seems to me the most probable will be
given in Chap. XV., namely that they existed primordially as glandular
hairs, or mere epidermic formations, and that their upper part should
still be so considered; but that their lower
* According to Nitschke ('Bot. Zeitung,' 1861, p. 224) the purple fluid
results from the metamorphosis of chlorophyll. Mr. Sorby examined
the colouring matter with the spectroscope, and informs me that it
consists of the commonest species of erythrophyll, "which is often met
with in leaves with low vitality, and in parts, like the petioles, which
carry on leaf-functions in a very imperfect manner. All that can be said,
therefore, is that the hairs (or tentacles) are coloured like parts of a leaf
which do not fulfil their proper office."
Dr. Nitschke has discussed this subject in 'Bot. Zeitung,' 1861, p. 241
&c. See also Dr. Warming ('Sur la Diffrence entre les Trichomes' &c.,
1873), who gives references to various publications. See also
Groenland and Trcul 'Annal. des Sc. nat. bot.' (4th series), tom. iii.
1855, pp. 297 and 303. [page 6]
part, which alone is capable of movement, consists of a prolongation of
the leaf; the spiral vessels being extended from this to the uppermost
part. We shall hereafter see that the terminal tentacles of the divided
leaves of Roridula are still in an intermediate condition.
The glands, with the exception of those borne by the extreme
FIG. 3. (Drosera rotundifolia.) Longitudinal section of a gland; greatly
magnified. From Dr. Warming.

marginal tentacles, are oval, and of nearly uniform size, viz. about
4/500 of an inch in length. Their structure is remarkable, and their
functions complex, for they secrete, absorb, and are acted on by various
stimulants. They consist of an outer layer of small polygonal cells,
containing purple granular matter or fluid, and with the walls thicker
than those of the pedicels. [page 7] Within this layer of cells there is an
inner one