CHAPTEE XIV 

 PLANTS 



Irritability. — As plants as well as animals are com- 

 posed largely of living matter, they as well as animals 

 manifest the physiological qualities of living matter, irri- 

 tability, assimilation, and reproduction, and the mani- 

 festation is governed by the same physical and chemical 

 principles. To understand, then, the life history of any 

 individual plant, its irritability, assimilation, and repro- 

 duction must be studied in relation to its structure, upon 

 which the peculiar way in which they manifest them- 

 selves is dependent. 



Plants are very irritable. Within the cell the proto- 

 plasm is in constant motion. In addition many of them 

 are able to move as independent organisms. Lower plants 

 through the activity of cilia move readily from place to 

 place. Higher plants are usually firmly fixed, but they 

 respond readily to external forces such as heat, light or 

 gravity, and bend the whole or a part of the body in the 

 direction of the acting force. Any number of illustra- 

 tions might be mentioned such as the twining of the 

 tendrils of peas, the snapping together of the valves of 

 venus fly-trap, the turning toward the sun of helio- 

 trope, the shrinking of a sensitive plant, and the droop- 

 ing of the leaves of oxalis. 



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