JACQUES LOEB 699 



The correctness of this reasoning (which is of importance for the 

 theory of polarity) can be demonstrated by experiments on the pro- 

 duction of roots in isolated leaves. When isolated leaves of Bryo- 

 phyllum calycinum are suspended sidewise in a moist atmosphere, 

 an abundance of roots is formed on the lower side of these leaves 

 and none or fewer on the upper side (see left half of Fig. 10). This 

 is due to the fact that roots commence to develop a little earlier 

 in the notches on the under than on the upper side of such a leaf and 

 that they grow in general a little more rapidly on the lower than on 

 the upper side. The more rapidly growing elements in a leaf inhibit 

 the growth of the more slowly growing elements of the same kind.^ 

 The correctness of this interpretation is demonstrated on the right 

 half of the photograph. The four leaves on the right side of Fig. 10 

 are the sister leaves of the four leaves on the left. The lower edge 

 of the leaves on the right side was cut off, thus preventing the 

 growth of roots on the lower side, since roots only grow out from 

 preformed buds in the notches of a leaf. In this case the roots on 

 the upper side grow out more abundantly. The photograph was 

 taken on the 25th day of the experiment. Figs. 11 and 12 demon- 

 strate another experiment of the same type. Fig. 11 shows that in 

 the intact leaves on the left half of the drawing, the roots grow out 

 sooner on the lower than on the upper side. The drawing was made 

 after 8 days. In the sister leaves on the right side of the drawing, 

 the lower edge of each leaf was cut off, thus preventing root forma- 

 tion on the lower side of the leaves. Practically no roots had formed 

 on the upper side of the leaves. Fig. 12 is a drawing of the same leaves 

 25 days later. In the leaves on the right side, where the notches on 

 the under side were cut off, the growth of roots and shoots on the 

 upper side is much more abundant than it is on the upper side of the 

 sister leaves on the left side of Fig. 12, where the growth on the 

 upper side was retarded or inhibited by the more liberal growth 

 on the under side of the leaves. The mass of shoots in each pair of 

 sister leaves was practically in proportion to the mass of the leaves. 



A striking demonstration of the principle that acceleration of root 

 (and shoot) formation in one part of a leaf inhibits the root (and shoot) 

 formation in the other part of the leaf is offered in Figs. 13 and 14. 

 The four leaves in Fig. 13 were put with their apices into water which 



