PRINCIPLES OF VEGETATION. s5S 



fome are opening and clofingat all times (infomuch that Linn^us, by 

 obferving their ftate, informs us we may know the hour of the day, and 

 hence calls them Flora's fVatch), and as others open only during night, 

 this caufe Teems ina.iequate to fo great variety of effefts. What is termed 

 the fletp of plants, is fuppofed to proceed from exhauftion of irritability, 

 and is moft obfervable in young plants, and in fpring. 



The Aiimqfaj or Sev.fit'vje Plant (of which Linnasus reckons forty- fix 

 fpecies) is the great inllance of irritability ; as, befide contracting its 

 leaves in the evening, it may be made to contraft them at any time, by 

 the touch of the hand, the fumes of volatile alkali, the electric fpark, 

 muik, or opium : and this contraction is fo powerful, that, on its taking 

 place, the leaves may be eafier broken than re-expanded j but the plant 

 will regain its natural (tare after an interval of from ten minutes to half 

 an hour. Immerfion in water does not produce thefe effects. Their ge- 

 neral progrefs is according to the force or quantum of ftimulus engaged 

 on the plant, affefting firft one leaf, then its fellow or pair on the ilalk, 

 then the pedicle or foot-ftalk, moving upwards, then all the leaves on 

 that branch, then the branch to the very ftcm of the plant. A fhaking 

 motion affefts the whole plant j but it is poflible to affe<5t the branches 

 and not the leaves. 



That plants are greatly afFefted by the impulfe of light, appears not 

 only from what we have faid when treating of light, but from the in- 

 flance of this very plant. A fenfitive plant, placed in a dark cave, was 

 twenty-four hours before it expanded, and then very partially j fufFered 

 no change at night or morning; but being placed in the open air, reco- 

 vered its natural ftate and irritability. Another, kept in a dark room 

 during fome hours after day- break, did not expand; but,reftored to the 

 light, expanded without delay. 



According to the ftrufture of plants they (hew their irritability in va- 

 rious manners : the Dionea Mufcipula^ or Venus's fly-trap, is fo called, 

 becaufe, if an infect enter its flower, defigning to tafte its fweets, the 

 leaves clofe upon hjm, and hold him beyond his powers to extricate him- 

 felf. 



Parafitical plants, which grow on other trees, as the mifTeltoe, or 

 which climb on other trees, as the ivy, and numberlefs others, are a curious 

 clafs of plants; many of which never drew nouriQiment from mother 

 earth, but from their fupporting tree, which they often ftrangle and de- 

 ftroy. In hot countries thefe are often fonoxioufly abundant, no art can 

 exterminate them. 

 Part Vi. No. 32. ' 3 O Veo^c- 



