METABOLIC GRADIENTS 83 



Fig. 21 (p. 76) are stages in the formation of leaves and 

 the developmental gradients appear to some extent in 

 them. 



In many plants new "adventitious" individuals 

 arise, either in nature or under experimental conditions, 

 from cells already differentiated as part of an individual. 

 In the liverwort, Metzgeria, new individuals may arise 

 either by division of the growing tip resulting in bifurca- 

 tion of the flat body, as shown in Fig. 35, a, a, or after 

 injury to, or removal of, the growing tip by a renewal of 

 division and growth in differentiated cells. Fig. 36 

 shows the cellular structure of the growing tip in a 

 well-developed individual and Fig. 37 the early stage 

 of a new individual formed from a differentiated 

 cell. In both figures the gradient in cell size is clearly 

 evident. 



Among the higher seed plants, as well as among lower 

 forms, the "adventitious" formation of new individuals 

 from differentiated cells occurs, as for example in the 

 begonias, where buds capable of producing new plants 

 arise under certain experimental and natural conditions 

 from the epidermal cells of leaves. The epidermal 

 cells which take part in the formation of such a bud 

 lose their differentiated, vacuolated condition, become 

 filled with protoplasm, like embryonic cells, and divide 

 rapidly. Fig. 38 is a surface view of the formation of 

 such a bud involving several epidermal cells, but centered 

 chiefly in parts of four cells, and Fig. 39 is a longitudinal 

 section through a bud formed from two cells. The 

 double contours in Fig. 38 show the thickened cellulose 

 walls of the original epidermal cells, the single contours 

 within them the cells formed by their repeated division, 



