STATOLITHS AND STATOCYSTS 597 



this stimulus-producing function to the starch-grains, which have a 

 relative density of about 1*5 ; but many other solid bodies, such as 

 calcium oxalate crystals, particles of silica, etc., might equally well act 

 as statoliths. This term may therefore be applied to all solid inclusions 

 that serve, by virtue of their weight, to transmit gravitational stimuli to. 

 the sensitive ectoplast. 



According to the theory first suggested by Noll, and subsequently 

 elaborated by the author and by Nemec, 301 the mode of perception of 

 gravitational stimuli in plants is analogous to the corresponding process 

 in animals. The investigations of Ernst Mach, Breuer, Chun, Delage, 

 Th. W. Engelmann, Verworn and others, long ago established the fact 

 that the so-called otocysts which are found in so many animals 

 especially in the lower groups have no connection with the sense of 

 hearing, but represent balancing organs, or, more properly, organs for the 

 perception of the " direction of gravity." For this reason Verworn 

 suggested the term statolith in place of otolith, the name previously 

 applied to the bodies which render gravitational stimuli effective by 

 virtue of their weight. 



Special gravitational sense-organs do not seem to be present in all 

 plants ; when they are absent, cells which perform totally different 

 principal functions may in addition serve for the perception of gravi- 

 tational stimuli, if they contain starch-grains or other bodies distinguished 

 by relatively high (or, on the other hand, relatively low) specific gravity. 

 At a higher level of adaptation, the geotropically sensitive members of 

 the plant-body are furnished with special geotropic sense-organs, a 

 striking instance of anatomico-physiological division of labour. 



In the Higher Plants each gravitational sense-organ typically com- 

 prises several usually a large number of sensory cells or statocysts. 

 Every statocyst consists of two essential portions, namely, first a 

 statolith-apparatus composed of a variable number of readily movable 

 or " falling " starch-grains, and, secondly, an ectoplast which is sensitive 

 to the pressure of these statoliths. When an organ containing stato- 

 cysts is in a condition of geotropic equilibrium, the pressure of the 

 starch-grains against the physically lower portion of the ectoplast re- 

 mains unperceived, or at any rate leads to no responsive movement. 

 But as soon as the part under consideration is displaced from its stable 

 position, the starch-grains fall against that portion of the ectoplast which 

 is now on the physically lower side of the cell ; a new and unfamiliar 

 state of stimulation is thereby produced, with the result that a geotropic 

 movement takes place, which brings the organ back into its former state 

 of equilibrium. In this case the stimulus need not consist of a shock 

 or jerk involving a sudden local deformation of the sensitive protoplasm, 

 such as was seen to be essential in the case of tactile organs. The 



