608 SENSORY SYSTEM 



are found in the highly reactive nodes (leaf-bases) of the Gea- 

 mineae. Many Grasses (e.g. Melica nutans) are in addition provided 

 with a well-developed one- or several-layered starch-sheath, which 

 is situated immediately within the inner epidermis of the sheathing 

 leaf base. 



F. Darwin has proved experimentally that, in certain Grass-seedlings, 

 geotropic sensitiveness is confined to the tip of the so-called cotyledonary 

 sheath ; this localisation of the perceptive faculty is especially charac- 

 teristic of the Paniceae. Nemec has found that, in all these 

 cases, the parenchymatous ground-tissue of the apical region contains 

 numerous starch-grains, which respond very readily to the influence 

 of gravity. 



The author's observations which have subsequently been confirmed 

 and extended by Schroder, Nuniec and Samuels 308 have shown, that 

 when floral organs exhibit geotropic sensitiveness, they are invariably 

 provided with statocysts, which usually take the form of a starch-sheath 

 surrounding the vascular strand or strands (e.g. in Hemerocallis fulva, 

 Funhia subcordata, Amaryllis robusta, Dictamnus Fraxinella, Azalea 

 pontica, Epilobium angustifolium, etc.). 



From what has been stated above, it is quite clear that the distribu- 

 tion, in space and time, of geotropic sensitiveness is correlated in a 

 remarkable manner with the presence of falling starch-grains. This 

 correlation might be regarded as accidental if it only occurred in a few 

 instances. In point of fact, a very large number of cases have been 

 described within recent years, which all support the statolith theory ; 

 indeed the anatomical evidence in favour of this theory may now be 

 considered conclusive. 



It is still unknown what structures, if any, act as statoliths among 

 the Bryophyta and Thallophyta. Nemec has, however, discovered 

 falling starch-grains in the geotropically sensitive stems and setae of 

 certain Mosses and Liverworts, and in the terminal cells of many 

 Moss-rhizoids. 300 The positively geotropic rhizoids of certain species of 

 Char a (C. fragilis, C. foctida, C. aspcra) contain, near their tips, minute 

 solid bodies of unknown chemical composition (Glanz-Korperchen), 

 which have been shown by Giesenhagen to respond to the action 

 of gravity. 310 Each group of these bodies, together with the asso- 

 ciated cytoplasm, seems to represent the statolith-apparatus of the 

 rhizoid. Possibly the terminal vacuoles, containing minute crystals, 

 which occur in geotactically sensitive Desmids [Closterium, etc.,], likewise 

 represent sensory organs. If plants of Caulerpa prolifera are darkened 

 for some time, their " leaves " put forth slender branches w T hich are very 

 strongly negatively geotropic. 311 The peripheral cytoplasm of these 

 branches contains rounded starch-grains at the exact point where 



