43O C. M. CHILD. 



secondary elongation at the basal end and these cells often show 

 a double gradient, i. e., the apical and basal ends are regions of 

 highest susceptibility and death progresses toward the middle or 

 a region somewhat below the middle of the cell. This appearance 

 of a secondary region of high susceptibility in the basal part of 

 a single cell where secondary growth is occurring is paralleled in 

 multicellular axes where secondary growth occurs in the basal 

 region as in Ectocarpus (Child, 'i6a), and a similar phenomenon 

 appears in many of the lower animals as a secondary growing 

 region at the basal posterior end, which may give rise to new 

 individuals (Child, '136) or to segments (Hyman, '16). Some- 

 times the gradient in the elongated cells of the basal region is 

 completely reversed. 



The rhizoicl of Griffithsia possesses a susceptibility gradient, 

 the apical end, the tip of the rhizoid, being the region of highest 

 susceptibility. In general the susceptibility of the apical end 

 of the rhizoid is considerably lower than that of the apical cell 

 of the vegetative axis. In these respects the rhizoid shows much 

 the same physiological relation to other parts as does the "stolon" 

 in the hydroid Tubularia (Child, '15^, pp. 91-92, 132-133). 



In Griffithsia, however, the degree of individuation (Child, 

 '156, Chap. IX.) is not high, the axial gradient is not very 

 permanently recorded in the protoplasm and therefore readily 

 undergoes modification under altered external conditions. It is 

 possible to eliminate or reverse the gradient experimentally in 

 various ways, e. g., by exposure to high temperature, and plants 

 or cells which have been injured or have been living under 

 unfavorable conditions show alterations of the primary gradient. 

 In general in the vegetative stages the more nearly normal the 

 physiological condition, the more distinctly and uniformly basi- 

 petal the gradient. An account of experiments along this line 

 in which one external agent is used to alter the gradient in 

 susceptibility to another is postponed to another time. 



In addition to these alterations certain agents in certain con- 

 centrations alter the susceptibility gradient to themselves. For 

 example in HgCl2 w/5OO,ooo the normal basipetal gradient ap- 

 pears. In w/5O,ooo, however, and in higher concentrations there 

 is more or less reversal in the apical region, i. e., the apical cell 



