1899.] NATUEAL SCIENCES OF PHILADELPHIA. 219 



THERMOTROPIC MOVEMENT OF THE LEAVES OF 

 RHODODENDRON MAXIMUM L. 



BY JOHN W. HARSHBERGER, PH.D. 



It is apparent from a iiuniber of recently published papers on the 

 reactions of protoplasm and, in particular, of various sensitive 

 plants to stimuli, that it will be necessary to reconstruct our views 

 to a considerable extent upon the subject of vegetable irrita- 

 bility. The movement of the chlorophyll granules in the cells 

 of leaves, exposed to too bright sunlight ; the movement of tendrils 

 in response to shocks, heat or the application of chemical substances, 

 show us that protoplasm reacts in essentially the same manner, 

 whatever plant is chosen for experimentation. 



The writer is not aware that any observations have ever been re- 

 corded on the movement of the leaves of Rhododendron maximum, 

 yet the movements of this plant are quite definite. If observa- 

 tions upon this ericaceous shrub be made during cold weather (the 

 recent zero weather affording fine opportunities for such study), 

 it will be found that the leaves are all bent down closely against 

 the stem, and are rolled inward tightly in a convolute man- 

 ner, one edge slightly overlapping the other, so that the upper 

 epidermis is alone presented to the action of the weather (fig. 1). 

 To assume this position, the petiole takes a sharp bend downward 

 through an angle of about seventy degrees. The lower side of the 

 petiole is puckered into transverse folds, when the inrolled and 

 hanging position of the leaves is assumed. The acute apex as 

 shown in one of the lower leaves (fig. 1) is slightly incurved. The 

 U-shaped bend of the petiole is more marked in the lower leaves 

 than in the upper. The leaves assume an extremely deep green 

 color, of a brownish hue, and appear as if frozen. Two objects 

 seem to be served by the hanging position of the leaves accompanied 

 by the folding of the lower surface of the blade, which is provided 

 Avith stomata, or transpiration openings. Firstly, the protection of 

 the lower surface of the leaf, thus ensuring the conservation of the 

 internal water of the plant. When the soil is hard frozen and the 



