146 Miss T. L. Prankerd. On the Irritability of the 



curve through 90° to bring itself into the vertical. Placed horizontally but 

 abaxially to the light, the frond will move into a vertical plane, at the same 

 time curving towards the incident light. If fronds are placed horizontally, 

 with their ab- or adaxial surfaces uppermost, the subsequent positions assumed 

 are shown in fig. 3. In the former case (A) gravity and epinasty act together ; 

 in the latter (B) they are balanced against each other, though gravity seems 

 to have the greater effect of the two, at least in the first phase, since an 

 upward curvature is often produced, though never approaching 90°. 



Although very little exact work has as yet been attempted, it is interesting 

 to note that both the latent* and reactionf periods for gravity are probably 

 considerably longer for ferns in general than angiosperms. In one experi- 

 ment Asplenium bulbiferum was compared with Sidalsia sp. The frond of the 

 one and the inflorescence axis of the other, neither unsevered from the parent 

 plant, were placed horizontally in the open at about 27° C. In 4 hours a 

 distinct geotropic curvature was visible in Sidalsia, and 1J hours more 

 proved sufficient to bring the axis back to the vertical, while no trace of 

 curvature was to be seen in Asplenium. And while I have made few exact 

 measurements^ of the reaction time for ferns, it has never been as low as 

 5| hours, except in Marsilia sp. at 18° C. The latent and reaction periods 

 vary greatly with temperature and perhaps with the season of the year. 



An attempt to express the relative effect of the last three factors and their 

 magnitude in the different phases of the life-history of the fern frond is made 

 in fig. 4, though the curves, with the exception of that of growth, are only 

 approximate. In the former case, the ordinates are proportional to heights 

 reached by the fronds on days represented by the abscissae. The gravity 

 curve rises with the growth of the frond, reaches a maximum in the 

 adolescent stage, and ceases probably before the frond is entirely uncurled. 

 It must, however, be understood that this, like other curves, attempts 

 to express the average of many observations, and does not necessarily hold 

 good exactly for any particular frond. The effect of light is represented by a 

 curve which begins after that of gravity, rises in the second phase to the 

 same height, and ceases probably soon after the beginning of the third phase, 

 and therefore after that of gravity. Finally, epinasty§ starts with the 



* The time elapsing between the first presentation of the stimulus and the beginning 

 of the response. 



t The time elapsing between the first presentation of the stimulus and the end of 

 the reaction. 



I The duration of these periods is now under investigation by my student, Miss 

 F. M. O. Waight. 



§ The subsequent relatively slight hyponasty is included here for convenience under 

 the term epinasty. 



