106 BOTANY. 



india-rubber tube 8 to 10 cm. long. Slip into the other end a small 

 glass tube 5 to 10 mm. in diameter, being sure to make the joints 

 water-tight. The ' ' root-pressure "will cause the water to rise into 

 the verticle tube. Note the effect of a change of temperature of the 

 soil. 



181. Supply of Energy to the Plant. — The work done 

 by a plant involves the expenditure of energy. In hystero- 

 phytes the decomposition of the chemical compounds ab- 

 sorbed by them affords a supply of energy fully or nearly 

 adequate for all their needs. In holophytes the case is far 

 different ; they absorb compounds of simple chemical con- 

 stitution supplying relatively little available energy, but in 

 their chlorophyll-stained cells they are able to arrest the 

 energy of the sunbeam, and divert it to the work of the 

 plant. Doubtless green plants derive some energy from 

 the decomposition of the compounds absorbed by them and 

 perhaps more from the heat to vrhich they are exposed, and 

 possibly to a slight extent from other sources, but the great 

 supply of energy is the light of the sun. It has been shown 

 experimentally that any other bright light, whether pro- 

 duced by lamps of various kinds or by the electric arc, 

 when of sufficient intensity, may be a source of energy for 

 green plants. 



PLANT MOVEMENTS. 



182. Living Things Move. — It is one of the essential 

 characteristics of living things that they move, although 

 " motility " and "life " are not synonymous. A complete 

 examination of the motility of plants would include the 

 many kinds of movements exhibited by protoplasm, 

 whether naked (as in zoospores) or enclosed within walls 

 of greater or less rigidity, and in addition the very slow 

 movements connected with growth and nutrition. These 



