TISSUE-DIFFERENTIATION 123 



ing or central nervous system ; that is, the impulse is 

 conveyed directly from a more or less diffusely 

 sensitive area to the tissue which reacts. 



Plant Movement. -- We ordinarily think of plants 

 as rooted fast in the ground. Nevertheless, apart 

 from the lower forms, mostly unicellular, which 

 move as do many Protozoa by means of the lashing 

 of flagella, a little observation will show that most 

 plants execute movements differing in intensity from 

 the sharp folding of the leaflets of the sensitive plant 

 to the gradual circling of the sunflower head as it 

 follows the sun from east to west during the day, or 

 the twisting of the climbing tendril. No such special- 

 ized tissue as muscle is to be found in plants, but all 

 such movements are to be attributed to changes in 

 the water content of masses of spongy tissue, such 

 that when the cells are full of water, the resistance 

 of the walls, or turgor of the tissue, makes it stiff and 

 erect, whereas the withdrawal of the water from the 

 tissues causes the leaves or other parts to become 

 flaccid or droop. A difference in the amount of 

 turgor on opposite sides of the stem will thus cause 

 the stem to bend to one side or the other according 

 to which side is under the greater tension. Sudden 

 movements are brought about by rapid alterations 

 in the turgor in localized areas under the influence 

 of external stimuli. 



Supporting Structures. In animals, as we have 

 seen, the skeleton consists of a framework, either 



