STEMS 



749 



When a growing tip is removed, the latent buds that develop and 

 replace it commonly are those nearest the apex. If these are removed, 

 those next below develop, and so on until no more buds remain. A 

 remarkable instance of this is seen in Phaseolus (fig. 1071), where buds 

 in the axils of the cotyledons may thus be induced to develop into shoots. 

 If all buds are removed, new 

 buds may organize, as from the 

 exposed part of the cambium 

 ring in a beech stump. The 

 factors operative in regeneration 

 are unknown, the current theory 

 relating the phenomena to cor- 

 relation; for example, a main 

 shoot is supposed to inhibit the 

 development of lateral shoots, 

 possibly because it utilizes the 

 available food through having a 

 better position or through making 

 an earlier start. The suppression 

 of the main shoot removes the 

 inhibition, and permits the lateral 

 shoots to develop. Such a con- 

 ception is of value chiefly in 

 stating the problem. 



Polarity. If a willow shoot 

 is placed in water, shoots de- 

 velop from the uppermost buds, 

 and roots originate near the base. FIGS. 1072, 1073. Polarity in a willow 



If the Shoot is inverted, roots de- ( Salix )- i7a, an ordinary erect cutting; 



1073, a cutting that has been grown in an 



Velop above and shoots below inverte d position; note that in each case the 

 (figs. 1072, 1073). Such a phe- roots (r) arise from the basal portion of the 



nomenon illustrates polarity, and stem ( root P le ) and the new shoots < from 



. ,. the apical portion of the stem (shoot pole). 



appears to indicate that plants _ After PFEFFER. 



and portions of plants possess 



root poles and shoot poles. Among other instances of polarity, there 



may be cited the appearance of shoots at the uppermost parts of exposed 



roots of Crataegus (fig. 722), of buds at the basal portions of severed 



leaves in Sansevieria (fig. 934), and of tubers on the upper ends of 



reversed shoots of the potato. 



