86 INDIVIDUALITY IN ORGANISMS 



region that the apical vegetative tip of the new plant 

 individual develops. 



In many woody plants the cut end of a stem or 

 branch develops a mass of wound tissue, the caUus, and 

 in this callus new buds arise independently of other 

 parts of the plant and become connected with them 

 secondarily. In all such cases the differentiation of the 

 vascular bundles which connect the new buds with the 

 old parts proceeds from the buds. Fig. 40 shows an 

 early stage of bud-formation in the poplar at the periph- 

 ery of a mass of callus on the cut end of a stem, and 

 Fig. 41, a later stage in which vascular connection with 

 other parts has been estabHshed. In such cases the 

 appearance of the new bud is the first step in the forma- 

 tion of the new individual; it is followed by the appear- 

 ance of a gradient in growth and differentiation from the 

 bud toward other parts. 



In isolated pieces of plants the formation of new 

 growing tips or the outgrowth of resting buds occurs 

 in certain more or less definite portions with relation 

 to the axes. The removal of the chief growing tip of a 

 stem results in outgrowth or altered growth of the 

 uppermost buds or branches. When these are removed 

 those lower down react, and so on. Evidently a gradient 

 in the capacity to respond or in the rate of response to 

 the altered conditions exists along the major axis, and 

 those buds or branches which react first dominate those 

 below them and prevent them from reacting in the 

 same way. 



In isolated pieces of the bilaterally symmetrical 

 liverworts, such as Marchantia (Fig. 23, p. 78), the 

 position of the new buds evidently represents the region 



