143 



On the Irritability of the Fronds of Asplenium bulbiferum, with 

 Special Reference to Graviperception. 



By T. L. Peankerd, B.Sc, F.L.S. Lecturer in Botany, University College, 



Eeading. 



(Communicated by Prof. W. M. Bayliss, F.R.S. Received March 10, — Revised 



November 11, 1921.) 



[Plate 3.] 

 1. Introduction. 



Our knowledge of the phenomena of irritability in vascular plants is 

 largely confined to Angiosperms. Very little work seems to have been done 

 on Pteridophytes ; and from his few observations on fern fronds, Darwin 

 [(1), p. 509] even doubted if they possessed any power of response to gravity. 



In a preliminary account (4) of the distribution of the statolith apparatus in 

 plants, I mentioned that it was present in young fern fronds, and I have since 

 found that this was partially known to Dehnecke (3). He referred to fern 

 fronds as possessing non-assimilating chlorophyll grains resting on their 

 physically lower cell walls ; but he did not connect these grains with the 

 perception of gravity, nor, so far as I am aware, have they ever been photo- 

 graphed or indeed noticed since. 



An investigation, therefore, of the physiology and cytology of young fern 

 fronds seemed advisable, in order to ascertain if, and how far, any connection 

 could be traced between them. The common plant Asplenium bulbiferum 

 proved a very suitable object for detailed study, though many of the observa- 

 tions described have been confirmed by work on other genera. 



2. The Growth and Movements of the Fronds. 



The plants used were grown in pots, generally under bell jars, in rooms 

 each of which had only one window ; thus subjecting them to one-sided 

 illumination the direction of which was known. A few control observations 

 were made on plants exposed to all-round illumination, and on others grown 

 in the dark. The latter were very little used, because it was desired to study 

 the behaviour of plants as far as possible under natural conditions. Many 

 series of observations extended over weeks or even months, where prolonged 

 darkness would have proved very deleterious, if not fatal. 



The position of a frond under observation was determined by measuring as 

 nearly as possible the angle made by the midrib with the horizontal. This 

 was done by a very simple, but effective instrument consisting of a transparent 



