282 FIRST YEAR COURSE IN GENERAL SCIENCE 



do not blossom under the artificial conditions in which they 

 are grown. These should not be called flowerless plants. 



320. The Influence of Light. One of the most re- 

 markable things about protoplasm is its sensitiveness to its 

 surroundings and its power to respond in certain definite 

 ways to varying outside conditions. For example, proto- 

 plasm is stimulated or roused to activity by light. If a bean 

 plant is grown in the dark, the stem will increase in length 

 more quickly than usual, but the leaves will be small and will 

 lack the usual green color. If a ray of light is admitted into 

 the dark. place, the stem will immediately grow toward this 

 light. The stems of house plants, which receive a compara- 

 tively small amount of light from windows, are more slender 

 and paler than the stems of those grown out-of-doors and 

 not shaded by neighboring plants. Moreover, plants grown 

 indoors always turn in one direction toward the light. 



321. Growth Movements. Only the younger, growing 

 parts of plants move under the influence of light, as it is 

 difficult for the older and somewhat rigid parts to change 

 their position. This response by movement is brought about 

 by the more rapid growth of the stem on one side than on the 

 other. When the bean plant is unequally lighted, growth is 

 most rapid on the side away from the brightest light, and 

 this makes the top curve toward the light. 



322. Another Response to Light. The behavior of 

 the tips of young shoots of English and Japanese ivies, which 

 are often seen covering the walls of stone or brick build- 

 ings, is quite the reverse of that of the bean plant. They 

 turn away from the light, though the leaves still face 

 the light. The ivies hold themselves in position by short 

 root-like growths on the stems, which attach themselves 

 to the rough stone. If the tip of the stem turned toward 

 the light, the hold of the vine on the stone would be 

 weakened. Different parts of the plant thus make differ- 

 ent responses to the stimulus of the light. 



