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PHYSIOLOGICAL BASIS OF POLARITY. II 



considerable mass of air roots while the stems kept in the dark 

 (Fig. 5) had formed practically no roots. In the dark the leaves 

 have a tendency to fall off but we are not ready to attribute the non- 

 appearance of roots on the stems kept in the dark purely to the inter- 

 ruption of the sap flow from leaf to stem. It is probably also partly 

 due to the interruption of the process of assimilation in the leaf. 

 This is suggested by the fact that we get the same inhibition or 



Fig. 6. Stems split longitudinally, each half stem having one apical leaf. 

 Stems on left, having their leaves covered with tin-foil, produce only few roots, 

 while stems on right, having their leaves exposed to light, produce in the same 

 time a considerable mass of air roots. Duration of experiment 27 days. 



suppression of root formation when we cover the leaf with tin foil in 

 which case the tendency of the leaf to fall off is less striking. Figs. 

 6 and 7 are illustrations of such an experiment after 27 days. There 

 was less dift'erence in the geotropic curvature of stems with leaves 

 in the light and in the dark than in the mass of roots produced. This 

 may be due to the fact that the geotropic curvature occurs at an 



