Biological Studies on Corymorpha. 409 



lus are in both cases strikingly similar in structure, being large, 

 with a relatively small amount of protoplasm and large vacuole, 

 and they bring about the bending of the stem by changing their 

 volume. In the seedling, the cells on all sides of the stem increase 

 in volume, but in those on the lower side the increase is greater 

 than in the cells on the upper side. In Corymorpha, a similar 

 differential between the upper and lower cells is established, 

 though this may not involve a change in volume of cells on both 

 sides of the stem. Just how it is established cannot be deter- 

 mined at present, for reasons which bring about an interesting but 

 not fundamental difference between the responses in plants and 

 Corymorpha. The increase in volume of the plant cells — their 

 growth — is permanent, because it includes growth of skeletal cell 

 walls which prevent the return of the cells to their previous size, 

 and it can be readily measured. The cells of Corymorpha have 

 no such walls, and can change their size without difficulty; their 

 growth is temporary and cannot be measured in the same way, 

 because the stem is liable to frequent non-geotropic changes in 

 length. The presence or absence of skeletal cell walls determines 

 whether the growth is to be permanent or transitory. This con- 

 sideration leads to the discussion of the geotropisms of such fixed 

 hydroids as the sertularians, some of which are known to be 

 geotropic, and all of which are provided with stout perisarcal 

 skeletons. 



The experiments of Driesch ('92) on species of Sertularella, 

 brought out the facts that the geotropic bending of the stem is not 

 general, but localized in the growing region at the end of the stem, 

 and that the growth which accompanies the bending is permanent. 

 My observations ('02) of Sertularia furcata and .S". argentea in 

 nature are in harmony with these results. "The San Francisco 

 colonies [of S. furcata] were growing on erect stalks of Phyllospa- 

 dix. The stems are short and project from all sides of the eel- 

 grass. Each stem leaves the eel-grass at an angle of about thirty 

 degrees, then bends quickly away so that for the most part it 

 makes an angle of seventy degrees with the stalk. The hydro- 

 thecae of the first, and often of the second pair as well, are not in 

 contact. Those of succeeding distal pairs are not only in contact 



