THE 



AMERICAN NATURALIST 



Vol. XLIV June, 1910 No. 522 



THE BOTANICAL SOCIETY OF AMERICA 1 



L THE NATURE OF PHYSIOLOGICAL 

 RESPONSE 



BY THE LATE PROFESSOR CHARLES R. BARNES 

 University of Chicago 



Livinc plants could hardly have been observed at all 

 without recognition of the fact that they are affected by 

 external conditions. These effects, when first observed, 

 were undoubtedly interpreted as mere conditions of liv- 

 ing, and our interpretation to-day has hardly gone be- 

 yond this. A moderate temperature, an adequate supply 

 of water, and the normal light are necessary for green 

 plants ; and when these conditions are suitable the gen- 

 eral behavior of the plant is affected. All its processes 

 have a healthy tone; it flourishes; and the factors which 

 operate to produce this condition are declared to be tonic. 



When the effects of external agents are studied more 

 closely, several peculiarities appear. Even these tonic 

 factors are seen frequently to produce quite limited 

 changes in behavior, though they may act simultane- 

 ously upon all parts of the body. Especially is this the 

 case when there is some sudden change in the intensity 

 or direction of these factors. Thus if the air tempera- 

 ture falls a few degrees on some spring day when the 

 crocuses are in bloom, the perianth segments promptly 

 curve inwards and close the flower. Or if the sunlight is 



