THE AXIAL GRADIENTS IN HYDROZOA. 



191 



stolon formation after two days in standing water, many of the 

 stolons being apical. Fig. 18 shows a case of apical transforma- 

 tion in this species. 



DIFFERENCES BETWEEN STOLONS AND STEMS. 

 There is usually no difficulty in distinguishing a stolon from 

 a stem. The stem supports itself and is relatively rigid, the stolon 

 is much less rigid and when it grows free in the water tends to 

 hang downward as its length increases, until it comes into con- 

 tact with a solid surface. Once in contact with a surface, the 

 perisarc of the stolon adheres to it and its further growth is along 

 the surface, but the stern does not attach itself to surfaces. It is 

 not true, however, that contact is an essential factor in the origin 



