144 



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



B 



C--- 



Fig. 1. — Instrument 

 for measuring the 

 angular height of a 

 fern frond. 



protractor (A, fig. 1) mounted between two strips of glass (B) fitted upright 

 into a slot in a wooden block (C). The two pieces of glass are held together, 

 and keep the protractor in place, by an ordinary letter- 

 clip (D) at the top. In order to bring the protractor 

 on a level with the frond, it can be lowered by simply 

 pressing it down from above, or raised by pushing it up 

 from below, at the same time opening and holding 

 down the clip to keep the supports in place. The 

 latter are made of glass, so that the whole frond should 

 be visible, and also to secure the easy adjustment of 

 the 90° line of the protractor against the frame of the 

 window — usually quicker than using a plumb line. 



Three phases may conveniently be distinguished in 

 the life-history of a fern frond ; the first or infant 

 phase when the apex of the frond is curled, the second 

 or adolescent phase, from the time when the first 

 leaflets appear beneath the apical coil, till the frond is 

 quite uncurled, when the third or mature phase is 

 reached. Growth cannot be measured as precisely in 

 ferns as in angiospermic stems and roots, owing to the circinate vernation and 

 the considerable individual variation in the process of uncurling. It frequently 

 happens in the middle of the second phase, that the apex is raised very quickly 

 {i.e., in about 24 hours), so that the recorded height shows a greater increment 

 than that due to growth in length. A fern frond, like the organs of angio- 

 sperms, shows a grand period of growth. When it first appears above the 

 soil it grows slowly — 0"5-l mm. or so a day. This increases till a young frond 



3- 10 cm. in height, i.e., in the second phase, may show a daily increment of 



4- 5 mm. In the third phase, growth slackens, and very gradually ceases 

 altogether. 



At least six types of movement are shown by fern fronds : — 



(1) Nutation. — This is exhibited most strikingly in the second phase of 

 growth, and the results I obtained for Asplenium are very similar to those 

 recorded for Nephrodium molle by Darwin [(1), p. 257]. 



(2) Bedipetality , by which I mean the tendency in growth for the different 

 parts of the mid-rib to be in the same straight line. This is sometimes 

 apparently secured in the adolescent stage by an autotropic movement of part 

 of the frond. 



(3) Sagging, due to the weight of the developing pinnae, is characteristic of 

 the end of the adolescent and beginning of the mature stage, and gives the 

 frond its final Graceful form. 



