30 THE NATURE OF LIFE 



Another illustration is afforded by the 

 well known "Praying Palm" of India, 

 which stands before a temple and is re- 

 vered because its head at evening is 

 bowed as if in prayer, while in the morn- 

 ing it is raised as if to greet the sun. 

 Scientific observation has shown that 

 change of temperature combined with the 

 action of gravitation causes the upper 

 side of the stem to grow more rapidly 

 during the day and the lower side to 

 grow more rapidly during the night, thus 

 causing the daily change of position. 



More striking is the kind of motion 

 due to muscular contraction. In biology 

 the word contraction is used in a special 

 sense. When we speak of the contraction 

 of a bar of iron we mean a diminution of 

 volume but when a muscle contracts 

 there is no alteration of volume, but only 

 a change of shape, whereby the muscle 

 grows shorter and thicker. This is also 

 true of the contraction of simple masses of 

 protoplasm, as in the one-celled organisms. 

 In these forms contraction is a much 



