474 BROOKLYN BOTANIC GARDEN MEMOIRS 



external cells of the organism come in contact with solid particles a 

 reaction follows. To afifirm, however, that the stimulus is mechanical 

 in nature does not explain anything, because we know nothing of 

 the nature of the so-called mechanical impulse. The bombardment 

 of organisms with electric waves (negative electrotropic response) 

 may be of the nature of a mechanical impulse and such may hold 

 true for other forms of radiant energy. Phototropic, thermotropic, 

 electrotropic and contact stimuli may not materially differ from one 

 another in the nature of their action on organisms, that is, in the sense 

 of acting in a mechanical manner on plants. The stimulation resulting 

 from such contact is apparently transmitted to the living zones which 

 induce definite adjustments or coordinations of the vital processes, 

 the nature of the response being determined by the nature of the 

 stimulus involved and the organs stimulated. The external cells 

 of a leaf, for example, coming in contact with an object would probably 

 react to the stimulus through the cuticle and exterior cell walls. The 

 reaction of plants to contact is probably one of the most primitive 

 forms of responses and quite universal in the vegetable kingdom.-'' 

 The nature of the response to contact resembles mostly that of touch 

 or at least a primitive and rudimentary form of this sense. 



There are several types of contact stimulation that have long been 

 recognized such as occur in the response of tendrils, tentacles, stamens, 

 etc., when brought in contact with different substances. These 

 reactions are associated with different types of irritability. Most of 

 the known reactions to contact such as are illustrated by tendrils, 

 etc., are closely associated with biological adapta<^ions, and as such 

 they have been subject to considerable modification. It is not at 

 all improbable, however, that these various types of reactions are 

 modifications and differentiations of a more simple and universal 

 form of contact response. The various forms of response movements 

 which are associated with irritability have been classified under the 

 so-called tropistic, nastic, and taxic reactions. The first type of 

 response is characterized by the movement of organs toward a definite 

 position in response to a stimulus, while nastic reactions are those 

 which are independent of the direction of the stimulus. Taxic reac- 

 tions are those shown by motile organisms. There are several different 

 types or manifestations of irritable phenomenona such as photo- 



"^^ Some experiments made with mucor in Petri dishes containing beef extract 

 and fine particles of quartz sand showed contact responses. Both series of Petri 

 dishes contained the same amount of beef extract, but in one the fine sand was added 

 for the purpose of obtaining contact with the mycelium. The sporangia were black 

 and mature in the Petri dishes without sand, while in the dishes containing the fine 

 sand, the aerial hyphae were much more elongated and the sporangia light in color 

 and less mature. 



