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. 

 If the shoot is inverted, roots de- 

 velop above and shoots below 

 (figs. 1072, 1073). Such a phe- 

 nomenon illustrates polarity, and 

 appears to indicate that plants 

 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. 



FIGS. 1072, 1073. Polarity in a willow 

 (Salix): 1072, an ordinary erect cutting; 

 1073, a cutting that has been grown in an 

 inverted position; note that in each case the 

 roots (r) arise from the basal portion of the 

 stem (root pole) and the new shoots (5) from 

 the apical portion of the stem (shoot pole). 

 After PFEFFER. 



