But plants possess, in obedience to various stimuli,
powers of movement, which are of manifest importance to them; for instance,
towards and more rarely from the light--in opposition to, and more rarely
in the direction of, the attraction of gravity. When the nerves and
muscles of an animal are excited by galvanism or by the absorption of
strychnine, the consequent movements may be called an incidental result,
for the nerves and muscles have not been rendered specially sensitive to
these stimuli. So with plants it appears that, from having the power of
movement in obedience to certain stimuli, they are excited in an incidental
manner by a touch, or by being shaken. Hence there is no great difficulty
in admitting that in the case of leaf-climbers and tendril-bearers, it is
this tendency which has been taken advantage of and increased through
natural selection. It is, however, probable, from reasons which I have
assigned in my memoir, that this will have occurred only with plants which
had already acquired the power of revolving, and had thus become twiners.
I have already endeavoured to explain how plants became twiners, namely, by
the increase of a tendency to slight and irregular revolving movements,
which were at first of no use to them; this movement, as well as that due
to a touch or shake, being the incidental result of the power of moving,
gained for other and beneficial purposes.
Pages:
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384