344 PLANT PHYSIOLOGY 



If a plant is exposed to too high a temperature, it does not re- 

 spond normally to stimuli and is said to be in a condition of heat 

 rigor. In a similar manner a state of cold rigor, of drought rigor, of 

 darkness rigor, etc., may be induced. If the state of rigor is not 

 too prolonged, the original irritability will be resumed when the 

 plant is restored to normal conditions. 



Characteristics of Irritability. — Irritability is characterized 

 first of all by a delayed response. The response to the change in 

 the environment is not immediate; it takes some time for it to be 

 made evident. Secondly, there is more energy used up or expended 

 in the response than was expended in the original stimulus. When 

 a boy is pricked with a pin, the amount of energy spent in push- 

 ing in the pin is very small compared with that with which the 

 response is shown when stored energy is released. Similarly in 

 plants, the amount of required energy expended by light on a 

 stem is less than that which the plant spends in bending toward 

 the light. Thirdly, the response of plants and animals to stimuli 

 is generally (but not always) of such a nature that it serves a 

 purpose. That is, in the course of evolution organisms which 

 responded as the present ones do, had a better chance of living 

 in the struggle for existence. The responses have thus had sur- 

 vival value. 



The delay of the response is the result of its three phases, — 

 the reception of the stimulus, the transmission of the stimulus, 

 and the final reaction or response proper. The receptive region 

 is not the same as the responding region. Animals receive stimuli 

 through the afferent nerve endings and respond through the mus- 

 cles and other parts of their effector systems. Plants may also 

 respond in a place very different from where the stimulus is re- 

 ceived. In the case of animals the receptive regions are generally 

 very specialized; the eyes receive light stimuli, the ears sound stim- 

 uli, etc. In plants such localized receptive regions for special stim- 

 uli are rare ; the entire surface is more or less sensitive to all stim- 

 uli. In other words, the reception is diffuse rather than localized. 



The question of the transmission of stimuli has always much 

 interested physiologists. In animals, the nervous system serves 

 to transmit stimuli, but in plants no analagous nervous system 

 has been recognized. Bose (1925) of India has asserted that the 

 conducting strands of protoplasm in the phloem are specialized 

 for the conduction of stimuli and have the same function as ani- 



