Fronds of Asplenium bulbiferum. 



151 



the inner ground tissue (at the right time and place, see above) is a statocyte, 

 and practically every chloroplast a statolith, so that we have a very complete, 

 if not the highest, type of apparatus. In good transverse sections cut from 

 fronds laid horizontally for an hour or two, the little heaps of chloro- 

 statoliths may easily be seen with a simple lens lying on the walls which 

 have been lowest. The statocytes are relatively large in cross-section, 

 though shorter than usual in length, being only about twice as long as they 

 are broad. 



4. Summary and Conclusion. 



From the foregoing facts, it will therefore be seen that the life-history of a 

 fern frond of Asplenium bulbiferum falls naturally into three periods, charac- 

 terised not only by external morphology, but by physiological response and 

 cy tological differentiation. This may be expressed in tabular form, thus : — 



Stage. 



Infant. 



Adolescent. 



Mature. 



Frond 



Curled 

 Beginning 

 Increasing 



Uncurling 

 At a maximum 



>» u 



Uncurled. 

 Ceasing early. 

 Absent. 

 )> 





Darwin's failure to recognise apogeotropism for ferns is accounted for by 

 the discovery of (1) the much greater reaction time; and (2) the com- 

 paratively early disappearance of geo tropic irritability in these plants. In 

 his experiment [(1), p. 509],* the older frond was probably losing its power 

 of response to gravity, and the period of horizontality (46 hours) would not 

 always be sufficient to induce curvature in the case of a frond " with the tip 

 still inwardly curled." Darwin does not give the exact stage of development 

 of this frond, nor the temperature of the experiment — points both of which 

 have been shown greatly to influence the time of reaction. 



Next in interest to the demonstration of the existence of both gravi- 

 perceptive power and the possession of statoliths by fern fronds, comes their 

 very close association. This has, of course, been noted for angiospermic 

 shoots (2), but in fern fronds it is more striking. In the first place, a 

 considerable part of the growth of the frond takes place after both its 

 statoliths and its irritability to gravity have disappeared, but when it is still 

 able to respond to light ; and, again, these disappearances are, as far as can be 

 ascertained, synchronous. 



* I demonstrated apogeotropism in the identical species used by Darwin — Nephrodium 

 molle— at Kew Gardens in 1916. 



