698 



PHYSIOLOGICAL BASIS OF POLARITY. II 



only on the basal side of the stem. The fact that no roots grow on 

 the upper side of the stem cannot, however, be dismissed with the 

 mere statement of an influence of gravity in this case. Fig. 4 shows 

 that an abundance of roots will be formed on the upper side of a 

 horizontal stem when the lower side is cut off. Why then will no 

 roots develop on the upper side of an intact stem suspended hori- 



FiG. 10. Leaves suspended sidcwise in air. On left side of photograph more 

 roots are formed on the lower than on the upper side of leaf; in the leaves on the 

 right side of the photograph the lower edge is cut off. In this case the upper 

 edge of the leaves produces the roots and shoots which otherwise would have de- 

 veloped below. Duration of experiment 25 days. 



zontally? The reason is that the roots grow more rapidly on the 

 lower side of a stem suspended horizontally and that on account of 

 this more rapid growth of the roots on the under side the growth 

 of roots on the upper side of the stem is inhibited. When we re- 

 move the under side of a stem this inhibitory effect disappears and 

 roots will grow from the upper side of the stem. 



