i8 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



old cellulose membrane. Figure 2 shows a case in which the locus of bud 

 formation involves most intensely the adjoining parts of four cells. The 

 more deeply shaded cells in the central region are filled with finely granu- 

 lar "embryonic" cytoplasm, and the optical density of this decreases from 

 the center peripherally, as indicated by the decreasing shading. The 



Fig. 4. — Five stages in development of an adventitious bud from an epidermal cell or 

 cells of flax hj'pocotyl: a, four derivatives of single epidermal cell; e, epidermis of hypocotyl; 

 Ip, leaf primordium of adventitious bud (from Crooks, 1933). 



peripheral unshaded cells are still more or less vacuolated, although they 

 have undergone division. The figure shows very clearly a radial gradient 

 system in rate of division indicated by cell size and a gradient in the same 

 direction of cytoplasmic character and content of the cells. Evidently the 

 intensity of the activation initiating formation of the bud decreases from 

 the central region peripherally. As development proceeds, the old cellu- 



