4o8 Harry Beal Torrey. 



those on the other side, it seems highly probable that the orientation 

 is not the result of muscular activity, but must be due to changes in 

 relative volume of the vacuolated endoderm cells on opposite sides 

 of the stem. And since these cells are exceedingly large, v^ith 

 excessively thin walls and almost no protoplasm, the changes in 

 volume appear to be due to changes in the turgidity of the cells. 

 This conclusion is borne out by the facts that the stem may not 

 only shorten w^ithout increasing its diameter, but may lengthen 

 yvhWe. actually increasing its diameter, results possible only through 

 a variation in the turgidity of the axial cells. A complete demon- 

 stration that muscles do not take part in the geotropic response 

 is lacking, because in spite of the numerous transverse cuts made 

 in the stem, the latter was still able to shorten (thickening at the 

 same time), showing that the muscles were not rendered entirely 

 impotent. But the slowness of the response and its occurrence 

 while the wounds gaped and the muscles on the upper side of 

 the stem were manifestly weaker than those on the lower side, 

 strongly support the view that they were not concerned in the 

 result. I think we may say that the muscles produce the 

 movements of the tentacles, proboscis, and all save the geotropic 

 movements of the stem, including shortening and possibly length- 

 ening (by means of the circular muscles) while the axial cells 

 cause the geotropic orientation as well as lengthening of the 

 stem.^ 



If organic growth is increase in volume,^ then the changes in 

 turgidity which affect the orientation and length of the stem must 

 be reckoned among growth processes, and as such they will be 

 found to differ in no fundamental respect from those growth pro- 

 cesses in plants and in all probability the fixed hydroids also which 

 accomplish the orientation of these organisms with reference to 

 gravity. This statement requires some comment. 



A comparison of the phenomena of geotropism in the stems of 

 plant seedlings and Corymorpha brings out points both of resem- 

 blance and difference. The cells reacting to the geotropic stimu- 



^The skeletal function of the axial cells, correlated with the alienee of a sup- 

 porting perisarc, will be considered in a subsequent paper. 

 ^Davenport, '97. 



