PHYSIOLOGY OF VEGETABLES. 



and other compound radiate ones. " In their forms 

 Nature seems to have delighted to imitate the radiant 

 luminary to which they are apparently dedicated, and 

 in the abscence of whose beams, many of them do not 

 expand their blossoms at all.' 5 



It has already been mentioned^ that the functions of 

 the leaves, in decomposing carbonic acid and deliver- 

 ing out its oxygen, were performed only in the sun- 

 shine. The most important functions of flowers are 

 also performed in the sunshine only. Most plants 

 vegetate but imperfectly in the shade, and scarcely at 

 all in perfect darkness. Trees, which are overgrown 

 by others around them, decline and finally die. 



Leaves and flowers are attracted by light, and prob- 

 ably have their vital functions stimulated by it. 



The pinnate leaves of the Sensitive-plant, Mimosa 

 sensitwa, and some other plants, have a most extraor- 

 dinary sensibility not only to light but to the touch of 

 any extraneous body or to any sudden concussion. By 

 even a gentle impression to one of the leflets they will 

 close together, one pair after another, and finally the 

 whole leaf will drop down to the stalk. 



" Weak with nice sense the chaste Mimosa stands, 

 From each rude touch withdraws her timid hands ; 

 Oft as light clouds o'erpass the summer glade, 

 Alarmed, she trembles at the moving- shade, 

 And feels alive through all her tender form 

 The whispered murmurs of the gathered storm ; 

 Shuts her sweet eyelids to approaching night, 

 And hails with freshened charms the rising light." 



Hedysarum gyraus has a spontaneous motion in its 

 leaves, independent of any external stimulus, even of 

 light, and only requiring a very warm atmosphere to 

 be performed in perfection. That it does not depend 

 on any motion of the atmosphere is proved by its con- 

 tinuing under a glass bell. 



These various phenomena depend no doubt on what 

 is termed vital irritability, which means a power in liv- 

 ing beings independent of any thing mechanical, by 

 which they act when certain agents are applied. 



