C. M. CHILD. 



regions are themselves indications of a differential susceptibility 

 in the stems. 



The staining with neutral red may be apparently uniform over 

 many millimeters of stem length, while considerable differences 

 in susceptibility, as indicated by the progress of disintegration 

 and decoloration may appear in the same region. As regards 

 the permeability of the perisarc in different regions to KNC, 

 nothing definite can be said, but the susceptibility gradients in 

 KXC are similar to those in neutral red. 



The differences in susceptibility are most marked near the 

 apical ends of stems and stolons, where the differences in rate 

 of growth are greatest within short distances. The growing 

 tip of a stem or a stolon is its most susceptible region, and the 

 susceptibility decreases rapidly from the tip basipetally within 

 a length of several millimeters to a few centimeters, according 

 to the species and the conditions of the stem. In the older 

 regions of stems and stolons differences in susceptibility are 

 much less marked, even over length of several centimeters, and 

 irregularities frequently appear, as in the older portions of the 

 thalli of alga 1 (Child, 'i6a, ' ' i6b], some regions dying earlier than 

 regions above or later than regions below. In these older parts 

 susceptibility is often the same in regions where differences in 

 thickness of perisarc are considerable, or it may be different in 

 regions covered by perisarc of equal thickness. 



In general the apical region of a growing stem which is to 

 develop into a hydranth is more susceptible than the apical 

 region of a growing stolon of the same colony, but no constant 

 differences have been observed between the older regions of 

 stems and stolons. That these differences between apical ends 

 of stems and stolons represent differences in physiological con- 

 dition is indicated by the fact, to be more- fully discussed in a 

 later paper, that the apical ends of stems can be reversibly 

 transformed into stolons by the action of low concentrations of 

 inhibiting agents. 



These data on susceptibility in stems and stolons are in agree- 

 ment with the facts of observation in that they show the regions 

 test growth activity to be most susceptible to lethal con- 

 trations, and the differences in susceptibility cannot be in- 



