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Show 4 DROSERA ROTUNDIFOLIA. CHAP. I. secondly, the power possessed by the leaves of rendering soluble or digesting nitrogeno?-s substances, and of afterwards absorbing them; thudly, the changes which take place within the cells of the tentacles, wh n the glands are excited in various ways. . . It is necessary, in the first place, to descnbe bnc:fly the plant. It bears from two or three to ~ve 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. r_rhe leaves are commonly a little broader than long, Fro. 2. ( D1·osera rotundifolia.) Old leaf viewed laterally; enlarged about fi ve timea. 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 thirtyone 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 arc short and stand upright, and their pedicels are green. Towards tho margin they become longer and longer and more inclined CHAP. I. STRUCTURE OF THE LEAVES. 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 t 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 some- times enter tru·e hairs.t 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 ag 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 colom·ing 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." t Dr. Nitschke has discussed this subject in 'Bot. Zeitung,' 1861, p. 241, &c. See also Dr. Warming ('Sur la Difference entre les Trichomes,' &c., 1873), who gives refe1·ences to various publications. See also Groonland and 'l'recul, 'Annal. des Sc. nat. bot.' (4th series), tom. iii. 1855, pp. 297 and 303. |