174 



POPULAR SCIENCE MONTHLY. 



SENSORY MECHANISM OF PLANTS. 

 By d. t. macdougal, 



FIRST ASSISTANT AND DIRECTOR OF THE LABORATORIES. NEW YORK BOTANICAL GARDEN. 



THE relation of the vegetal organism to its environment has de- 

 manded a much more generalized type of sensory action than 

 that of the animal, Thus but few species of plants have developed spe- 

 cial perceptive organs. The sensory functions are exercised by ex- 

 tended regions of the body, yet the delicacy of appreciation of differ- 

 ences in the intensities of external forces is not surpassed by that of the 

 animal. Thus no plant has sensory organs for the reception of light- 

 stimuli, yet, as a matter of regulation of their main function of food- 

 building, leaves react to differences in intensity far beyond the range 

 of the unaided human eye. Special tactile organs are differentiated in 

 tendrils and in certain '^carnivorous' species and 'sensitive' plants, 



Fig. 1. Surface View of Cells of Perceptive Region of the Columna of StyHclium 

 gramini/olium. After Habeklandt. 



Fig. 2. Longitudinal Section THROUGH Fig. 3. Epidermal Cell of the Perceptive 

 A Single Papilla of an Epidermal layer of Tendril of Entada scandens. 

 Cell of the Columna of Styiidium 

 firaminifolium, which is Sensitive to 

 Contact. 



in which members are adapted to a narrow and unusual non-typical 

 purpose. Here also great delicacy and accuracy is obtained, and the 

 contact or weight of a body inappreciable to the sense of touch of any 

 known higher animal may act as a stimulus. This refinement of re- 

 action in undifferentiated tissues is quite remarkable. As a further 

 instance it may be cited that leaves of certain seedlings are capable of 

 appreciating an intensity of light equal to .00033 of a standard candle. 



