To test the correctness of this view, a small plant, growing in light peaty earth in a pot (only 4 1/2 by 4 1/2 inches outside measure) was copiously watered, and then kept without a drop of water in the hothouse. Two of the upper tubers were beforehand uncovered and measured, and then loosely covered up again. In a fortnight's time the earth in the pot appeared extremely dry; but not until the thirty-fifth day were the leaves in the least affected; they then became slightly reflexed, though still soft and green. This plant, which bore only ten tubers, would no doubt have resisted the drought for even a longer time, had I not previously removed three of the tubers and cut off several long rhizomes. When, on the thirty-fifth day, the earth in the pot was turned out, it appeared as dry as the dust on a road. All the tubers had their surfaces much wrinkled, instead of being smooth and tense. They had all shrunk, but I cannot say accurately how much; for as they were at first symmetrically oval, I measured only their length and thickness; but they contracted in a transverse line much more in one direction than in another, so as to become greatly flattened. One of the two tubers which had been measured was now three-fourths of its original length, and two-thirds of its original thickness in the direction in which it had been measured, but in another direction only one- third of its former thickness. The other tuber was one-fourth shorter, one-eighth less thick in the direction in which it had been measured, and only half as thick in another direction.
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 dried and pressed.*
Utricularia nelumbifolia (Organ Mountains, Brazil).--The habitat of this species is remarkable. According to its discoverer, Mr.