HOW PLANTS AND ANIMALS RECEIVE STIMULI 193 



How Plants and Animals receive Stimuli. — In the simplest 

 plant and animal cells which live by themselves there are no spe- 

 cialized parts which are especially fitted to receive outside stimuli. 

 The amoeba, for example, is influenced 

 by temperature, food, and other stimuli, 

 but it has no sense organs. Some tiny 

 plant-like animals (or animal-like plants) 

 such as eugle'na have a tiny structure 

 called an eyeshot, which seems to be more 

 sensitive to light than other parts of the 

 cell. 



The more complex single-celled ani- 

 mals, as Paramecium, have parts of the 

 cell (cilia) more sensitive to touch than other parts. Animals 

 and, to a lesser degree, plants, as they become more complex in 

 structure, tend to have special parts set aside to receive stimuli. 

 These special parts of complex animals are called sense organs. 



Responses in Plants and Animals. — The tropisms may be listed 

 as foUows : 



Euglena. Point out nu- 

 cleus, vacuole, chloroplasts, 

 and flagellum. The eyespot is 

 the dark granular mass above 

 the vacuole, near the top. 



Plants 



and 

 Animals 



Animals 



' Phototropism or response to light (see page 16) 

 Geotropism or response to gravity 

 Hydrotropism or response to water 

 Thigmotropism or response to contact 

 Chemotropism or response to chemical substances 

 Thermotropism or response to temperature changes 



^ Galvanotropism or response to electricity 

 Rheotropism or response to water currents 

 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. 



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- 



