12 Dr. Gardner on the Action of Light upon Vegetables. 



This result alone, is abundantly sufficient to decide the ques- 

 tion, and show the total inactivity of the tithonic rays in produ- 

 cing these vegetable movements. 



(32.) That the bending is not due to heat, appears from the 

 following considerations. The action is greatest in those parts of 

 the spectrum which give evidence of least heat. The axis is ap- 

 proached on one side by plants from the red, orange, yellow and 

 green, and by those from the violet and lavender on the other, 

 which is a phenomenon, altogether inexplicable on the supposi- 

 tion, that heat is the active agent. 



Plants shut from the light of an Argand lamp, by a plate of 

 copper foil, do not incline to the warm metal. 



Finally, the moonbeams, even without condensation, are ca- 

 pable of producing extensive bending, in one or two hours. This 

 result is conclusive of the question, for no trace of caloric can be 

 found in the moon's light. 



(33.) As far, therefore, as the presence of heat can be deter- 

 mined by thermoscopes, or the tithonic rays, by argentine com- 

 pounds, and the union of chlorine and hydrogen, we are justified 

 in concluding that the movements of plants are effected by a to- 

 tally different agent. Light only, remains in the spectrum, so 

 far as we know, and to it, therefore, I refer the motions under 

 consideration. 



(34.) This conclusion is of deep interest, inasmuch as it is the 

 first case of a movement, perceptible to the eye, being traced to 

 the unaided action of light. That this imponderable produced 

 molecular changes was readily admitted, but its influence in 

 bringing about palpable movements of considerable extent, has 

 never been suspected. In the irritability of the iris, physiologists 

 have always seen the influence of nervous matter ; but in plants 

 no such agent exists to complicate the phenomenon, and there- 

 fore the action is due to light only. 



In this newly discovered property, light is also more closely 

 assimilated to the other imponderables — for both heat and elec- 

 tricity are capable of producing palpable motion. 



III. Some applications of the preceding facts, fyc. 



(35. ) Numerous applications to vegetable physiology will sug- 

 gest themselves to the reader, but it is my purpose to treat only 

 of the following. 



