474 BROOKLYN BOTANIC GARDEN MEMOIRS 
external cells of the organism come in contact with solid particles a 
reaction follows. To affirm, 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 ina 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 adaptations, 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- 
20 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. 
