142 



Light has no 

 effect on the 

 irritability. 



Life and irrita- 

 bility extin- 

 guished toge- 

 ther. 



Irritability 

 strongest in 

 the morning 



This irritabi- 

 lity probably 

 common to al 

 plants, 



IRRITABILITY OF VEGETABLES. 



in the water. Having taken it out, I found by repeated 

 trials, that the exudation followed irritation, as it appeared 

 to me, nearly as before; so that I could not find any percep- 

 tible difference in its irritability after it had been exposed to 

 this cool temperature. 



Thus the irritability of vegetables does not appear to suffer 

 by a sudden transition from a high to a moderate tempera- 

 ture, or to be diminished in proportion to it: though the 

 preceding experiment shows, that, when their irritability has 

 been heightened by a very hot atmosphere, and they are 

 placed for an instant in a cold one, it is perceptibly dimi- 

 nished. 



Light is well known to act as a stimulus on plants : but I 

 did not find greater marks of irritability in the lettuce or sow- 

 thistle when surrounded by sunshine, than when in the shade. 



i tried the effects of the solar light concentrated by a lens 

 on these two plants ; but it did not produce any irritation, 

 so as to cause the exudation of the usual fluid, though it 

 scorched them, when sufficiently intense. 



I pulled up some whole plants of lettuce and sowthistle, 

 and also stripped off some branches, and left them to wither 

 on a table in my room in summer. About ten hours after I 

 irritated them where the effect would be most visible, and 

 obtained some slight marks of irritation. I then placed the 

 stalks of one or two of these plants in water ; and after some 

 time I found they recovered from their apparent death, and 

 began to vegetate afresh. A little time after I attempted to 

 irritate some others, that were still more withered ; but they 

 exhibited no exudation. I then put them in water like the 

 former, but they never recovered. Thus in plants life and 

 irritability appear to become extinct together. 



I tried to irritate plants of lettuce and sowthistle, growing 

 ill the same ground, at various hours of the day and night ; 

 and I found their exudation most energetic in the morning, 

 when the sun had risen, and their flowers were fully ex- 

 panded. 



The property, that lettuce, sowthistle, and spurge have, 

 of giving out a milky fluid, or their peculiar juice, when any 

 of their more succulent, parts are irritated, appears to me, to 

 render the existence of irritability in plants unquestionable. 



It 



