Page:An introduction to physiological and systematical botany (1st edition).djvu/82

52 order to suit the constitutions of different tribes of plants destined for different parts of the globe. It cannot but act as a stimulus to the living principle, and is one of the most powerful agents of Nature upon the vegetable as well as animal constitution. Besides this, however, various mechanical causes may be supposed to have their effect; as the frequently spiral or screw-like form of the vessels, in some of which, when separated from the plant, Malpighi tells us he once saw a very beautiful undulating motion that appeared spontaneous. This indeed has not been seen by any other person, nor can it be supposed that parts so delicate can, in general, be removed from their natural situation, without the destruction of that fine irritability on which such a motion must depend. We may also take into consideration the agitation of the vegetable body by winds, which is known by experience to be so wholesome to it, and must serve powerfully to propel the fluids of lofty trees; the passage, and evolution perhaps, of