ALTERATION OF THE AXIAL GRADIENT. 



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215 



but is basipetal when present. Often the apical cell itself shows 



a distinct basipetal gradient. Several cells from the third or 



fourth on usually show a distinct basipetal intracellular gradient, 



but with the beginning of the secondary basal elongation of the 



cell a basal region of high susceptibility usually appears, and 



some of these elongating cells show a 



double gradient, a region near the mid- 



dle being the least susceptible. In these 



elongated cells of the more basal levels 



of well-developed axes the intra- cellu- 



lar gradients may differ from cell to cell 



and from time to time, even though the 



general axial gradient remains essentially 



unaltered. 



Not infrequently rhizoids develop on 

 these more basal elongated cells even at 

 a distance of several cells from the basal 

 end of the axis, and in such cases the 

 rhizoid may appear at any level, even 

 close to the apical end of the cell, while 

 the latter is still, at least physically, a 

 part of the axis. In cases where such 

 well-developed rhizoids are found on 

 such cells it is of course impossible to 

 determine the relation between the in- 

 tracellular gradient and the point of 

 origin of the rhizoid at the time when 

 the rhizoid arose, and the possibility al- 

 ways exists that the intra-cellular gradi- 

 ent may have undergone alteration since 

 the origin of rhizoid. Rhizoid development is primarily a feature 

 of the basal end or the more basal regions of the axes of algae 

 (Tobler, '06), and as such we might expect to find it associated 

 with regions of low susceptibility, i. e., of low metabolic rate. 

 The alterations in the intracellular gradients of the older more 

 basal cells of Griffithsia suggest the possibility that even where a 

 rhizoid arises from the middle or the apical region of such a cell 

 this region may be the region of lowest metabolic rate in that 

 particular cell. 



