A slice was cut from one of these shrivelled tubers [page 441] and
examined. The cells still contained much water and no air, but they
were more rounded or less angular than before, and their walls not
nearly so straight; it was therefore clear that the cells had
contracted. The tubers, as long as they remain alive, have a strong
attraction for water; the shrivelled one, from which a slice had been
cut, was left in water for 22 hrs. 30 m., and its surface became as
smooth and tense as it originally was. On the other hand, a shrivelled
tuber, which by some accident had been separated from its rhizome, and
which appeared dead, did not swell in the least, though left for
several days in water.
With many kinds of plants, tubers, bulbs, &c. no doubt serve in part as
reservoirs for water, but I know of no case, besides the present one,
of such organs having been developed solely for this purpose. Prof.
Oliver informs me that two or three species of Utricularia are provided
with these appendages; and the group containing them has in consequence
received the name of orchidioides. All the other species of
Utricularia, as well as of certain closely related genera, are either
aquatic or marsh plants; therefore, on the principle of nearly allied
plants generally having a similar constitution, a never failing supply
of water would probably be of great importance to our present species.
We can thus understand the meaning of the development of its tubers,
and of their number on the same plant, amounting in one instance to at
least twenty.
UTRICULARIA NELUMBIFOLIA, AMETHYSTINA, GRIFFITHII, CAERULEA,
ORBICULATA, MULTICAULIS.
As I wished to ascertain whether the bladders on the rhizomes of other
species of Utricularia, and of the [page 442] species of certain
closely allied genera, had the same essential structure as those of
Utricularia montana, and whether they captured prey, I asked Prof.
Oliver to send me fragments from the herbarium at Kew. He kindly
selected some of the most distinct forms, having entire leaves, and
believed to inhabit marshy ground or water. My son Francis Darwin,
examined them, and has given me the following observations; but it
should be borne in mind that it is extremely difficult to make out the
structure of such minute and delicate objects after they have been
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