THE AXIAL GRADIENTS IX HYDROZOA. 197 



end of the hydranth-stem axis, but is itself an axis, a gradient, as 

 has IK-CD demonstrated in various ways. Like new hvdranth- 

 stem axes, it originates from a bud and the high end of its gradient 

 constitutes a growing tip instead of a hydranth. At the stolon 

 tip synthesis ot protoplasm, growth and cell division apparently 

 keep pa< e with each other as in the growing tip of a plant. >< that 

 differentiation doe-, not occur, but when the tip i- raised to a 

 higher ph\>i. 1 level by change in external condition- it 



loses the capacity for unlimited growth and differentia ner 



or later into a hydrunth. 



The demonstration that the stolon is a somewhat inhibited a\i- 

 oi vM'adicnt throws light on many otherwise pu//ling feature- of 

 the l>eha\ior of isolated pieces of hydroids and hydioid colonies. 

 The -toloii gradient may be inhibited, either l.\ external con- 

 ditions or by it- nearness to a dominant hydranth region il'hild. 

 '15. pp. 91-2). Consequently a stolon axis ma\ arise at a cut 

 -in lace which is near vigorous hydranth regions .,,nl it may also 

 aii-e under conditions which prevent formation <>r maintenance 

 o| hxdtaiiths. Pieces in deep standing water nia> produce 

 stolons, those in shallow, or frequently changed \\aier hxdranth- 

 stein axes. Pieces may produce first stolons, then hydranth- 

 stem axes or vice versa, with slight differences in conditions of 

 cult ure. 



Three conclusions based on the evidence presented in this and 

 earlier papers of this series and on the work of l.und and otht r- 

 i.int in this connection: first, the stolon as \\ell a- the 

 liV Irani h -stem represents an axis, a polarity, a gradient ; -e. <.nd, 

 the stolon -radient represents primaril\- lo\\er le\el> of |)h\-io- 

 logical activity than the hydranth-stem gradient and re\ei>ibli- 

 transformation in either direction is pos>ilile and does not 

 necessarily inxolve re\ - ersal of polarity : third, hydranth-stem or 

 ^tolon axis may be determined according to condition-, 1,\ the 

 qtiantitatixe differential or local action of environmental fa< tors 

 independently of their specific constitution. The- lu^ions 



afford a ph\ r-iological basis for the interpretation of data at hand 

 and for further investigation and control of polarity. 



