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Show 264 DROSERA ROTUNDIFOLIA. CHAP. XI. though with considerable force and with a hard object, the tentacle does not bend. The plant is thus saved from much useless movement, as during a high wind the glands can hardly escape being occasionally brushed by the leaves of surrounding plants. Though insensible to a single touch, they are exquisitely sensitive, as just stated, to the slightest pressure if prolonged for a few seconds ; and this capacity is manifestly of service to the plant in capturing small insects. Even gnats, if they !est on the glands with their delicate feet, are quickly and securely embraced. The glands are insensible to the weight and repeated blows of drops of heavy rain, and the plants are thus likewise saved from much useless movement. The description of the movements of the tentacles was interrupted in the third chapter for the sake of describing the process of aggregation. This process always cmn1nences in the cells of the glands, the contents of which first ·become cloudy ; and this has been observed within 10 s. after a gland has been ex.cited. Granules just resolvable under a very high power soon appear, sometimes within a minute, in the ?ells b~neath the glands ; and these then aggregate Into minute spheres. The process afterwards travels down the tentacles, .being arrested for a short time at ~ach transverse partition. The small spheres coalesce Into larger spheres, or into oval, club-headed, threador necldac~-like, or otherwise shape<l masses of protoplasm, which, suspended in almost colourless fluid exhibit incessant sponta:o.eous changes of form. Thes~ frequently coalesce and again separate. If a gland has been powerfully excited, .all the cells down to the ~ase of th~ tentacle are affected. In cells, especially If filled WIth dark red fluid, the first step in th.e CHAP. XI. GENERAL SUMMARY. 265 process often is the formation of a dark red, bag- · like mass of protoplasm, which afterwards divides and undergoes the usual repeated changes of form. Before any aggregation has been excited, a sheet of colourless protoplasin, including grclnules (the primordial utricle of Mohl), flows round the walls of the cells ; and this becomes more distinct after the contents have been partially aggregated into spheres or bag-like masses. But after a time the granules are drawn towards the central masses and unite with them; and then the circulating sheet can no longer be distinguished, but there is still a current of transparent fluid within the cells. Aggregation is excited by almost all the stimulants which induce move1nent; such as the glands being touched two or three times, the pressure of minute inorganic particles, the absorption of various fluids, even long in1mersion in distilled water, exosn1ose, and heat. Of the many stimulants tried, carbonate of an1monia is the n1ost energetic and acts the quickest: a dose of T:r4 1 400 of a grain ("00048 mg.) given to a single gland suffices to cause in one hour wellmarked aggregation in the upper cells of the tentacle. The process goes on only as long as the protoplasn1 is in a living, vigorous, and ~xygenated condition. The result is in all respects exactly the same, whether a gland has been excited directly, or has received an influence fron1 other and distant glands. But there is one important difference : when the central glands are irritated, they transmit centrifugally an influence up the pedicels of the exterior tentacles to their glands ; but the actuul process of aggregation travels centripetally, from the glands of the exterior tentacles down their pedicels. rrhe ex- . citing influence, therefore, which is transmitted from |
