PROBLEM 4. Hoiv Flams Respond to Swimli 



".o<; 



Fig. 283 Mimosa, or Sensitive Plant. When the photograph at the left ivas taken, the 

 plant had been undisturbed for several hours. The whole plant was slightly jarred 

 and, after a few 7mnutes, again photographed. Which parts responded to the stivmlus? 

 How is the response fnade? (general biological supply) 



them. See Figure 284. The pitcher plant, 

 common in many swamps, is also an in- 

 sect-eating plant but it merely traps in- 

 sects without directly responding to their 

 touch. Once the insect is caught all these 

 plants secrete juices which digest the 

 prey. 



Animal and plant behavior. Animals 

 and plants have little in common in their 

 behavior. This is to be expected since 

 their structure is so different. But they 

 have one thing in common. They are liv- 

 ing, that is, they consist of protoplasm 

 which is irritable. Therefore, they all re- 

 spond to stimuli in the environment, even 

 though the stimuli may be very different. 



Plants, in general, have a large variety 

 of tropisms. Parts of the plant turn to- 

 ward or away from a variety of stimuli 

 such as light, gravity, water, and chem- 

 icals. Certain plants, such as the tulip 



in its "sleep" movements, the insect- 

 eating plants, and some others have re- 

 sponses other than tropisms, but these 

 are the exceptions. Plants have nothing 

 like a nervous system nor even nerve 

 cells. Plant responses are slow because 

 movements toward or away from the 

 source of a stimulus are by unequal 

 growth on opposite sides of the stem or 

 root, or by changes in turgidity of cells. 

 When it is a matter of growth, it takes 

 hours (or days) for the response to be- 

 come evident. The growth hormones, or 

 auxins, play a part in such movements. 



As you can see, behavior in plants is 

 far simpler than behavior in all but the 

 simplest animals. Since plants lack a nerv- 

 ous system, and lack even nerve cells, 

 they cannot have the variety of responses 

 shown by animals. Nor can they show 

 any but very simple behavior. 



