120 PHYSIOLOGY OF VEGETABLES. 



ed upward with considerable force, amounts to several 

 tons. ; 



In animals, the hlood is propelled around its curcuit 

 by the powerful muscular contractions of the heart and 

 arteries. It is also solicited by the action of the ex- 

 treme vessels. Mr. Knight supposes that the silver 

 grain, being susceptible of quick contraction and ex- 

 pansion by heat and cold, has an agency in propelling 

 the sap by pressing upon the vessels. 



It has been supposed that the propulsion of the fluids 

 in vegetables takes place in a manner similar to that 

 of animals, i. e that they are in some way acted on by 

 the coats of the large vessels, and that they are like- 

 wise drawn up by the action of the extreme vessels of 

 the leaf. The spiral coats of the vessels have been 

 suspected of having an agency in this business. " In 

 some of these, when separated from the plant, Mal- 

 pighi 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 sup- 

 posed, that parts so delicate can, in general, be remov- 

 ed from their natural situation, without the destruction 

 of that fine irratibility, on which such a motion must- 

 depend." Smith. 



Observation. I was once examining some Eel-grass, in which 

 these spirals are numerous and may be dra*vn out to a great 

 length ; but my object at this time was not to observe them. The 

 grass was moist ft?om the salt water and laid in a heap, various- 

 ly broken and torn asunder, on a table in a dry, warm room. 

 I\vas presently surprised by the appearance of numbers of white 

 filaments in a very brisk vermicular motion, and which I actually 

 took for little worms. They seemed to raise themselves frequent- 

 ly in end, and being bent at various angles, turned rapidly round 

 in one direction a while, then changed and turned in the opposte, 

 making at the same time various other animated movements. Put- 

 ting them in the microscope, I found they consisted of the spiral 

 coats, which, from the manner in which they had been drawn out, 

 were twisted, and when drawn quite asunder had, consequently, 

 twisted together into an elegant little cord, which being very sus- 

 ceptible of changes of volume by alternate dryness and moisture, 

 had been put in motion by the fluctuating vapour arising from 

 the moist grass. 



