THE MOVEMENTS AND HABITS OF PLANTS. 29 



light, and the cotyledons of course remained expanded ; 

 both pots were now placed close together in the middle 

 of the room, and the cotyledons which had been exposed 

 to the sun immediately began to close, while the others 

 opened ; so that the cotyledons in the two pots moved in 

 exactly opposite directions while exposed to the same 

 degree of light. 



We found that if seedlings, kept in a dark place, were 

 laterally illuminated by a small wax-taper for only two 

 or three minutes at intervals of about three quarters of 

 an hour, they all became bowed to the point where the 

 taper had been held. We felt much surprised at this 

 fact, and, until we had read Wiesner's observations, we 

 attributed it to the after-effects of the light ; but he has 

 shown that the same degree of curvature in a plant may 

 be induced in the course of an hour by several interrupt- 

 ed illuminations lasting altogether for twenty minutes as 

 by a continuous illumination of sixty minutes. We be- 

 lieve that this case, as well as our own, may be explained 

 by the excitement from light being due not so much to 

 its actual amount, as to the difference in amount from 

 that previously received ; and in our case there were re- 

 peated alternations from complete darkness to light. In 

 this and in several of the above-specified respects, light 

 seems to act on the tissues of plants almost in the same 

 manner as it does on the nervous system of animals. 



INFLUENCE OF GRAVITATION UPON PLANTS. 



Gravitation excites plants to bend away 

 from the center of the earth, or toward it, or 

 to place themselves in a transverse position with respect 

 to it. Although it is impossible to modify in any direct 

 manner the attraction of gravity, yet its influence could 

 be moderated indirectly, in the several ways described in 



