148 



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



period, especially if the conditions are such that the reaction period is long, 

 it.'will probably never reach the vertical, but remain at some angle, continuing 

 its growth in the same straight line. In most cases when this occurs, there 

 is a certain amount of oscillation before the frond becomes fixed in the oblique 

 position. 



Kg. 5 illustrates the life-history of an actual fern-frond moving under 

 gravitational stimulus. The dotted line of growth is plotted as before, the 





Pot horizontal 



Pot upright 



Pot horizontal 



90 









BO 









70 









EO 









5 0° 









40 









30 









20 









10° 



i 









20 30 9 19 2 9 9 19 29 8 18 24 



Oct. Nov. Dec. Jan. 



Fig. 5. 



other curve showing the rise of the frond, where the ordinates represent 

 degrees. When the frond was only a centimetre or so in height, the pot was 

 placed horizontally, and the frond slowly rose till it was at an angle of 60° 

 with the horizontal. Like most fronds under these conditions, it never 

 reached the vertical, probably in order to avoid the darkness due to the 

 proximity of the soil. When the pot was placed upright, the frond attained 

 the vertical position. Again placed horizontally, the frond rose to an angular 

 height of 70° and then dropped. This was not due to the cause just 

 explained, since the same thing has occurred in experiments when the pot 

 was upright (see below). The frond is seen to oscillate for some time, and 

 finally to take up a position at an angle of 45°, the recorded deviations from 

 this being well within the limits of experimental error. 



Text-fig. 6 is from a tracing made on glass of the actual position occupied 

 by a fern frond on the recorded dates. A complete movement through 90° 

 having been obtained by previously placing the pot horizontally, it was on 



