244 HISTORY OF BOTANY 



There are two stimuli in nature to which plants are 

 peculiarly sensitive, viz. gravity and light. The typical 

 root is positively geotropic and the shoot axis negatively 

 so, while dorsiventral leaves are diageotropic. These 

 fundamentals were made clear to us by Knight early 

 in the nineteenth century, and the discovery of the 

 sensitive protoplasm by Von Mohl enabled physiologists 

 to recognise that " vehicle of irritation " of whose existence 

 Knight could not convince himself. The next questiorj 

 that arose was, what was it that stimulated the proto- 

 plasm ? An answer to this question was offered in 1900 

 by Noll, Haberlandt, and Nemec. These investigators 

 held that each organ that responded to the stimulus of 

 gravity possessed certain sensory cells or " statocysts," 

 the external layer of whose protoplasm was peculiarly 

 sensitive to the impacts of minute granules, such as 

 starch grains, to which the name " statoliths " was given. 

 The theory is expressed by Haberlandt in the following 

 words : 



" When an organ containing statocysts is in a 

 condition of geotropic equilibrium, the pressure of the 

 starch grains against the physically lower portion of the 

 ectoplast remains 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." 



The transference of the starch grains from the original 

 to the new position occupies some time, and, as they 

 continue falling on the new sensitive area, the stimulus 

 increases in intensity. This period, which lasts from 

 five to twenty minutes according to Haberlandt, is called 

 the "presentation time," that is, the period of time 



