Fronds of Asplenium bulbiferum. 



147 



inception of the frond, and is represented towards the end of the second 

 phase higher than gravity or light, since its effect at this time is more pro- 

 nounced than either, and seems to dominate the situation. It ceases at a 

 period somewhere near the complete uncurling of the frond, but which varies 

 greatly with the individual. Indeed, the last curve must be taken as even 

 more approximate than either of the others, since epinasty is very variable, 

 both in amplitude and the time of appearance and disappearance of its 

 remarkable expression towards the end of the second phase. 



3- d phase 



Fig. 4. — For explanation see text. 



It is important for our purpose that the time at which geotropic irritability 

 ceases should be ascertained as nearly as possible, but in actual practice this 

 is very difficult, as the hyponastic curve is apt to simulate that due to 

 geotropism, and the matter is complicated by the individual variability of 

 the frond, which is most apparent in the very unstable adolescent stage. It 

 seems at least certain that growth continues after the cessation of geotropic 

 irritability, the frond being at this time only 70-80 per cent, of the length 

 finally attained. 



The fern frond is in many ways the biological equivalent of the angio- 

 spermic shoot, and, like it, will bring itself back into the vertical should it be 

 displaced from this position. It accomplishes this by passing the vertical 

 and swinging like a pendulum backwards and forwards till it remains upright 

 in a manner similar to that well known for flowering plants, and originally, I 

 believe, described by Darwin [(2), p. 508]. This takes place if the position 

 of a frond is altered in the first, or in the earlier part of the second phase. 

 Should, however, a frond be placed horizontally towards the end of this 



