410 Harry Beal Torrey. 



for half their length but tend much more strongly toward the upper 

 side of the stem than do the proximal hydrothecae. This would 

 seem to be an instance of the effect of gravity upon the direction of 

 hydranth buds. The farther the stems diverge from the vertical, 

 the more closely do the hydrothecae of each pair crowd each other 

 on the upper side of the stem" (p. 66). "The habit of the San 

 Francisco colonies of iS". argentea seems to be controlled in an 

 interesting fashion by gravity. The branches are borne on all 

 sides of the stems, which were fastened by their bases to the per- 

 pendicular side of a shore boulder. Each stem had curved up- 

 ward, so that while the basal portion was nearly horizontal, the 

 terminal fourth or fifth was approximately vertical. In this 

 terminal vertical portion the branches and the hydrothecae on 

 them were arranged symmetrically with respect to the axis of the 

 colony; and in this region the axis of the colony and the lines of 

 force of gravity were parallel. At the base, where they were not 

 parallel, branches and hydrothecae were oriented with respect to 

 the force of gravity alone. Both hydranth and branch buds, as 

 well as the stem, thus appear to be more or less negatively geo- 

 tropic, the hydranths always being borne on the upper sides of 

 the branches; the latter grow away from the center of the earth 

 but never become parallel with the main stem" (p. 68). 



It is not difl&cult to explain why the geotropic bending is at the 

 end of the stem. It is only at the growing tips of stems and 

 stolons that the perisarc is dissolved. Elsewhere the cells of the 

 coenosarc may contribute perisarc, but are ordinarily unable to 

 dissolve it. At the tip, then, either by means of muscular activity 

 {cf. the free Cerianthus) or, more probably, by growth processes 

 similar to those taking place in Corymorpha — it is not yet de- 

 termined which — the stem assumes an orientation which is 

 temporary at first, becoming permanent only when the harden- 

 ing of the perisarc about it prevents further bending. 



Enough has been said to show that no fundamental distinctions 

 can be made between the geotropism of permanently fixed, tem- 

 porarily fixed and permanently free organisms, such as plants, 

 hydroids, anemones, protista. Any theory, then, which seeks to 

 offer a thoroughly satisfactory interpretation of geotropism in one 



