The Green Plant 493 



carried on through the medium of the stems, leaves, roots, and 

 seeds. 



Sensation and Motion. Although the plant has not the 

 specialized senses that animals possess, nevertheless it is sensitive 

 to external influences, especially to light, heat, and moisture. No- 

 tice how leaves gradually turn toward the light when not facing 

 it, how the tiny roots reach out toward water, and how susceptible 

 plants are to lack of heat. Some plants are so sensitive that they 

 wilt and droop when merely touched. The ability of plants to 

 respond to stimuli is limited; nevertheless, they possess sensation 

 and the power to move. By the exercise of these functions they 

 adjust themselves to their surroundings. 



Respiration. Respiration is an absolutely necessary function 

 in plants as well as in animals. Upon it depends the oxidation of 

 food, the release of energy, and, in short, the very life of the 

 plant itself. 



In respiration oxygen from the air passes into the intercellu- 

 lar spaces of the leaves through the small openings, called stomata, 

 on the lower surface. Then it is absorbed through the cell mem- 

 branes and enters into the sap, which transports it to the different 

 parts of the plant body where it comes into contact with digested 

 food and oxidizes it. In this process energy is released and carbon 

 dioxide and water are produced, as in animals. These products 

 are returned to the leaves where they are excreted. Although the 

 leaf is considered the main organ of respiration, as a matter of 

 fact this process occurs in all living parts of plants. 



Some oxygen also enters the plant through the root hairs and 

 through the lenticcls, or minute openings in the bark. A little 

 carbon dioxide is likewise thrown off by these organs. 



In these ways oxygen is carried to all the living cells of plants, 

 and the energy needed by the cells is supplied through the oxida- 

 tion of nutrients that are stored there. Thus we see that respira- 

 tion serves the same purpose in plants as in man and animals. 



One reason why the farmer tills the soil thoroughly before 

 he sows the seeds is that seeds must have oxygen in order to 

 germinate. By stirring the soil the farmer introduces air into it, 

 and, under proper conditions of warmth and moisture, the oxygen 



