JACQUES LOEB 385 



Fig. 8) has no pigment in its notches, but the pigment is visible in 

 the stem and especially in the small leaves formed on the upper side 

 of the stem. It is needless to say that while only a few drawings are 

 given the experiments were made on a large number of specimens, all 

 yielding the same result. Fig. 9 shows the distribution of the pig- 

 ment in the cortical layer of a piece of stem connected with a leaf. 

 While the purple pigment is a convenient indicator for the dis- 

 tribution of sap, it is not the cause of the growth of roots and shoots 

 in a leaf. This is obvious from the fact that growth of roots and 

 shoots takes place in isolated leaves when kept in the dark or when 

 dipping in water, although no noticeable formation of the purple 

 pigment takes place under these conditions. 



SUMMARY AND CONCLUSIONS. 



1. When leaves of Bryophyllum calycinum are suspended in moist 

 air in a vertical plane and side wise, roots and shoots are formed 

 exclusively or predominate in the notches on the lower side of the 

 leaves. When pieces of stems of the same plant are suspended hori- 

 zontally in moist air, roots develop on the lower side of the stem, with 

 the exception of the extreme basal end where they may develop on 

 both sides. 



2. The writer has suggested in a preceding paper that this directive 

 influence of gravity on the arrangement of the regenerating organs 

 may be due to the combination of two factors. The first factor is 

 gravity, which causes a slightly greater collection of sap on the lower 

 side of these organs, and as a consequence roots develop a little more 

 quickly on the lower than on the upper side. The second factor is of 

 an inhibitory character inasmuch as quite generally organs which grow 

 out first, or which grow quickly, have a tendency to retard or inhibit 

 the growth of similar organs in other places. 



3. The writer was able to prove the action of this inhibitory factor 

 by cutting off the lower edges of leaves suspended sidewise in a vertical 

 plane or the lower halves of stems suspended in a horizontal plane 

 (in moist air). In this case roots developed as abundantly on the 

 upper side of these organs as they otherwise would have developed 

 on the lower side. 



