CERTAIN GENERAL CIL\RACTERISTICS 



17 



consist primarily in an activation of some sort which decreases in intensity 

 from the center. As the bud develops, the more rapid growth of the physi- 

 ologically central region brings about elongation, usually more or less ver- 

 tically to the surface on which the bud appears, and the radial differential 

 becomes longitudinal (Fig. i). Usually the central region becomes the 

 apical end of the new axis; but in some buds the center may change its 

 position, the axis may become asymmetrical, or the original center may 

 divide into two or more, as in dichotomous branching and various sorts of 

 appendages. 



Figs. 2 and 3. — Development of an adventitious bud from epidermal cells of a Begonia 

 leaf. Fig. 2, surface view; Fig. 3, longitudinal section. The old cellulose membranes are indi- 

 cated in Fig. 2 by double lines; the membranes of new cells, by single lines. In Fig. 3 the heavier 

 line indicates an old cellulose membrane undergoing resorption (from Regel, 1876). 



Adventitious buds which develop from epidermal cells of parts of plants ' 

 which are physically or more or less physiologically isolated from active 

 vegetative tips provide almost diagrammatic examples of the gradient 

 system and its transformation. Buds of this sort developing from epider- 

 mal cells of the leaf blade or petiole in certain species of Begonia, after iso- 

 lation of the leaf, were described and figured by Regel (1876). Before the 

 bud appears, the epidermal cells of the leaf possess cellulose membranes 

 and large vacuoles, have ceased to divide and grow, and under the usual 

 conditions would never divide or grow again. In short, they have all the 

 characteristics of differentiated cells. The locus of a bud may be a single 

 one of these cells, or it may involve several of them. The beginning of 

 bud formation is indicated by disappearance of the differentiated vacuo- 

 lated condition, accumulation of cytoplasm, and rapid division within the 



