SECT. I. EXCITABILITY. 441 



sun ; but the third, though tall, was pale and sickly. 

 Hence it is upon the principle of the exclusion of 

 light that plants are blanched, as in the case of the 

 blanching of Celery, which is sometimes termed also 

 etiolation. 



The direction and luxuriance of the branches The direc 

 depend also on the presence and action of light, as branches, 

 is particularly observable in the case of hot-house 

 plants, the branches of which are not so conspicu- 

 ously directed, either to the flue in quest of heat, 

 or to the door or open sash in quest of air, as to 

 the sun in quest of light. Hence also the branches of 

 plants are often more luxuriant on the south than on 

 the north side ; or at least on the side that is best 

 exposed to light. 



The position of the leaf is also strongly affected The posi- 

 by the action of light to which it uniformly turns f^ 

 its upper surface. This may be readily perceived 

 in the case of trees trained to a wall, from which 

 the upper surface of the leaf is by consequence 

 always turned ; being on a south wall turned to the 

 south, and on a north wall turned to the north. 

 And if the upper surface of the leaf is forcibly 

 turned towards the wall and confined in that po- 

 sition for a length of time, it will soon resume its 

 primitive position upon regaining its liberty, but 

 particularly if the atmosphere is clear. Bonnet 

 tried to retain a leaf in its inverted position by 

 means of twisting the leaf-stalk ; but it was always 

 found to untwist itself again in the course of a 



