ALTERATION OF THE AXIAL GRADIENT. 22Q 



environmental conditions a gradient of decreasing metabolic 

 activity extends basally. The rhizoid itself is a similar gradient, 

 and the internal condition which favors rhizoid-development is 

 apparently a low metabolic rate, though external conditions 

 may determine whether it actually develops or not. It is, in 

 fact, like the stolon of the hydroid, Tubularia, a secondary 

 gradient, originating as a local region of increased metabolic 

 activity in consequence of partial physiological isolation (Child, 

 '156, pp. 91, 92), in which external factors probably, at least 

 often, play a part (Child, '156, pp. 132-134). The rhizoid, in 

 short, represents a secondary, more or less subordinate gradient, 

 originating at what may be called a certain metabolic level in 

 the axis or between certain metabolic limits and probably 

 developing in relation to certain external factors. This con- 

 ception agrees well with the fact that the susceptibility of the 

 rhizoid, even at its tip, is usually lower than that of the apical 

 region of the main axis, while even at its base, its susceptibility 

 is usually higher than that of the level from which it arose. 



Rhizoid-formation is not necessarily limited to the extreme 

 basal end of the axis, for as metabolic rate decreases with ad- 

 vancing age of the cells, other levels may attain a condition 

 which permits rhizoids to develop, and rhizoids may therefore 

 appear first at the base and later rhizoid-formation may progress 

 acropetally along the physiologically older portions of the axis, 

 as it actually does in various species of algae (Tobler, '06) and to 

 some extent and rather irregularly in Griffithsia. As regards 

 their relations to the primary axial gradient the roots of the 

 higher plants are probably similar to the rhizoids (Child, 'i5&, 

 pp. 156-163), though their function is of course different. 



Assuming the correctness of this conception of the simplest 

 plant axis, we should expect to find that in cells isolated from a 

 preexisting axis new apical cells would arise from the region of 

 the cell possessing the highest metabolic rate and rhizoids from 

 the region of lowest rate, or at least from lower metabolic levels 

 than the apical cell. So far as determined, the susceptibility 

 gradients in the forms developed from isolated cells agree in 

 general with this expectation. In every case from my own 

 culture, where only the earlier stages of new axial development 



