120 Introduction to Botany. 



92. Greater Sensibility of Growing Members. — In com- 

 paring these photographs, it is seen that the younger 

 portions of the stem and the younger leaves are more 

 responsive to the reversed position or to unilateral forces. 

 It appears that, in the older members, the protoplasts may 

 be less sensitive to external forces, and that it is a more 

 difficult matter for the older and somewhat rigid parts to 

 change their positions. 



93. Cause of Movements. — The movements which we 

 have just observed are due to a more rapid growth on one 

 side of the leaves and stem than on the other. When the 

 plant is inverted, the cells on the lower side of the petioles 

 and leaf blades and on the lower side of the stems increase 

 in size more rapidly than those of the upper side, and there 

 is accordingly a general bending of these parts upward. 

 When the plant is illuminated on one side more than on 

 another, growth is most rapid on the side remote from 

 the source of greatest illumination. When a plant which 

 has made a one-sided growth under such circumstances is 

 again placed so that it is equally illuminated on all sides, 

 its parts draw back toward their normal position, provided 

 they are not too old for growth ; but in order that this may 

 occur, growth must take place on one side more than on 

 another, although all sides are now equally illuminated. 

 It appears that in such cases gravity is used as a guide, 

 and that the protoplasts persist in causing unilateral growth 

 until the stems and leaves stand in their normal positions. 

 The experiment illustrated in Fig. 51 is very instructive, 

 for it teaches that when the directive influence of all ex- 

 terior forces is removed the protoplasts are no longer able 

 to determine a definite position for the stems and leaves. 



94. Heliotropism. — As we have seen in the study of 

 leaves, the interception of sunHght is of vital importance 



