METABOLIC GRADIENTS 85 



Fig. 39 from the upper part at the free surface of the 

 leaf downward. These gradients are evidently the 



FIGS. 38-41. Origin of adventitious buds in seed plants: Fig. 38, 

 surface view and Fig. 39, section of bud arising from differentiated epi- 

 dermal cells of leaf of Begonia (from Regel); Figs. 40, 41, development 

 of bud in callus (from Simon). 



visible expression of gradients in metabolic activity, the 

 smallest, most protoplasmic cells indicating the region of 

 most intense activity, and it is from this most active 



