Insectivorous Plants eBook

This eBook from the Gutenberg Project consists of approximately 462 pages of information about Insectivorous Plants.
excited by contact with fine fibres, others by contact with bristles, others with a flat or a creviced surface.  The sensitive organs of Drosera and Dionaea are also specialised, so as not to be uselessly affected by the weight or impact of drops of rain, or by blasts of air.  This may be accounted for by supposing that these plants and their progenitors have grown accustomed to the repeated action of rain and wind, so that no molecular change is thus induced; whilst they have been rendered more sensitive by means of natural selection to the rarer impact or pressure of solid bodies.  Although the absorption by the glands of Drosera of various fluids excites move- [page 366] ment, there is a great difference in the action of allied fluids; for instance, between certain vegetable acids, and between citrate and phosphate of ammonia.  The specialised nature and perfection of the sensitiveness in these two plants is all the more astonishing as no one supposes that they possess nerves; and by testing Drosera with several substances which act powerfully on the nervous system of animals, it does not appear that they include any diffused matter analogous to nerve-tissue.

Although the cells of Drosera and Dionaea are quite as sensitive to certain stimulants as are the tissues which surround the terminations of the nerves in the higher animals, yet these plants are inferior even to animals low down in the scale, in not being affected except by stimulants in contact with their sensitive parts.  They would, however, probably be affected by radiant heat; for warm water excites energetic movement.  When a gland of Drosera, or one of the filaments of Dionaea, is excited, the motor impulse radiates in all directions, and is not, as in the case of animals, directed towards special points or organs.  This holds good even in the case of Drosera when some exciting substance has been placed at two points on the disc, and when the tentacles all round are inflected with marvellous precision towards the two points.  The rate at which the motor impulse is transmitted, though rapid in Dionaea, is much slower than in most or all animals.  This fact, as well as that of the motor impulse not being specially directed to certain points, are both no doubt due to the absence of nerves.  Nevertheless we perhaps see the prefigurement of the formation of nerves in animals in the transmission of the motor impulse being so much more rapid down the confined space within the tentacles of Drosera than [page 367] elsewhere, and somewhat more rapid in a longitudinal than in a transverse direction across the disc.  These plants exhibit still more plainly their inferiority to animals in the absence of any reflex action, except in so far as the glands of Drosera, when excited from a distance, send back some influence which causes the contents of the cells to become aggregated down to the bases of the tentacles.  But the greatest inferiority of all is the absence of a central organ, able to receive impressions from all points, to transmit their effects in any definite direction, to store them up and reproduce them. [page 368]

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Insectivorous Plants from Project Gutenberg. Public domain.
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