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Show 274 DROSERA HOTUNDIFOLIA. CHAP. XI. In the ninth chapter the effects of the absorption of yan.o us a lka 1 o 1 · ds an ·c l certain other subst. ances were (1 e scn'b e d. Althou{Yb h some of these are poisonous, yet as several , wh l.ch act powerfully on the nervous sys. tem of ann· naI s , produce no effect on Drosera, wd e mady 1hn fe· r that the extreme sensibility of the glan s, an t eu. power. of transn1itting an influence .t o do ther. pa. rts of the leaf, causing movement, or mod1fie secretion, or aggregation, does no~ depend on ~he prese~ce of a d.ff d element alhed to nerve-t1ssue. One of the 1 use ' . · · h t remarkable facts is that long Im1ners1on 1n t e mo·s f the cobra-snake does not 1. n t h e 1e as t po1son o . . check, but rather stimulates, the spontaneous move-ments of the protoplasm in the c~lls of the tentacl~s. Solutions of various salts and ac1ds behave very differently in delaying or . in quite arresting the su.bsequent action of a solution of phosphate o~ ammonia. Camphor dissolved in water a~ts a~ a stu~ulant, as do s1nall doses of certain essential oils, for they cause rapid and strong inflection. Alcohol is not a stimulant. The yapours of camphor, alcohol, chloroform, sulphuric and nitric ether, are poisonous in mode~ately large doses, but in small doses serve as narcotics. or anccsthetics, greatly delaying the subsequent a~tion of meat. But some of these vapours also act as stimulants, exciting rapid, almost spasmodic movements ~n the tentacles. Carbonic . acid is likewise a narcotiC, and retards the aggregation of the protoplasm when 0arbonate of ammonia is subsequently given. The first access of air to plants which have been immersed in this gas sometimes acts as a stimulant and induces moyement. But, as before remarked, a special pharmacopreia would be necessary to describe the diversified effects of various substances on the leaves of Drosera. In the tenth chapter it was shown that the sensii ive- CHAP. XI. GENERAL SUMMARY. 275 ness of the leaves appears to be wholly confined to the glands and to the immediately underlying cells. It was further shown that the motor impulse and other forces or influences, proceeding from the glands when excited, pass through the cellular tissue, and not along the fibro-vascular bundles. A gland sends its motor impulse with great rapidity down the pedicel of the san1e tentacle to the basal part which alone bends. The impulse, then passing on wards, spreads on all sides to the surrounding tentacles, first affecting those which stand nearest and then those farther off. But by being thus spread out, and from the cells of the disc not being so m ueh elongated as those of the tentacles, it loses force, and here travels much more slowly than down the pedicels. 0 wing also to the direction and form of the cells, it passes with greater ease and celerity in a longitudinal than in a transve1·se line acrosR the disc. The impulse proceeding from the glands of the extreme marginal tentacles does not seem .to have force enough to affect the adjoining tentacles ; and this may be in part due to their length. The impulse from the glands of the next few inner rovvs spreads chiefly to the tentacles on each side and towards the centre of the leaf; but that proceeding from the glands of the shorter tentacles on the disc radiates almost equally on all sides. When a gland is strongly excited by the quantity or quality of the substance placed on it, the motor impulse travels farther than from one slightly excited; and if several glands are simultaneously excited, the impulses from all unite and spread still farther. A.s soon as a gland is excited, it discharges an i1npulse which extends to a considerable distance; but afterwards, whilst the gland is secreting and absorbing, the impulse suffices only to keep the same tentacle T 2 |
