RESPONSES OF THE PLANT 423 



Responses of plants and animals. The responses which plants 

 and animals make to certain definite stimuli are called tropisms. 

 Such responses may be either positive or negative, and appear 

 to be mechanical behaviors. They may be listed as follows : 

 Phototropism or response to light 

 Geotropism or response to gravity 

 Plants Hydrotropism or response to water 



and < Thigmotropism or response to contact 

 Animals Chemotropism or response to chemical substances 

 Thermotropism or response to temperature changes 

 . Galvanotropism or response to electricity 

 Animal / R neo ^ r °pi sm or response to water currents 



I Anemotropism or response to air currents 

 The response of roots to gravity, the growth of stems toward the 

 source of light, the opening of some flowers in the daytime and 

 others only at night, the climbing of plants by means of tendrils or 

 other organs stimulated by touch, are a few of the many examples 

 which might be mentioned. 



Practical Exercise 1. Make a list of all tropisms that you have ever seen 

 plants or animals exhibit. 



Some parts of the plant are more sensitive. While a plant 

 as a whole is sensitive to stimuli of different kinds, it is certain that 

 some parts are more sensitive than others. For example, experi- 

 ments show that in the root an area of not more than one milli- 

 meter in length is most sensitive to gravity, as the turning response 

 takes place there. Some tips of stems show a similar sensitive- 

 ness, and so do certain parts of growing leaves. 



Self-Testing Exekcise 



Check in your workbook the correct statements : 



T. F. 1. The tip of the root responds most readily to gravity. 



T. F. 2. Euglena has an eyespot which is sensitive to light. 



T. F. 3. Geotropism is response to the stimulus of gravity. 



T. F. 4. Phototropism is response to the stimulus of chemical sub- 

 stances. 



T. F. 5. Rheotropism is response to water current. 



