JACQUES LOEB 



383 



leaves suspended in air than in water. The leaf dipped into water 

 {i.e. the leaf with a normal ratio of dry to fresh weight) formed no 

 noticeable quantity of pigment while the other leaf with less water 

 formed a considerable quantity. This difference is constant. A 

 second factor necessary for the appearance of red pigment in the 

 leaf is light. When leaves are suspended in moist air but kept in the 

 dark they form no noticeable amount of red or purple pigment (Fig. 6) 

 though they form roots and shoots. 



Fig. 8. Two sister leaves, one (to the right) connected with a piece of stem, the 

 other (to the left) detached from the stem. Though both leaves are suspended in 

 moist air only the detached (left) leaf is full of pigment, while the leaf in connection 

 with stem is free from the purple pigment, which collects in the stem and in the 

 leaves of the bud formed by the stem. 



The most important fact for our problem is the distribution of sap 

 in leaves suspended sidewise and in a vertical plane in moist air. 

 In such leaves a collection of sap is found in the vessels close to the 

 notches of the lower side of the leaves where the roots and shoots 

 develop (Fig. 7). 



