346 BOTANY PART i 



at a certain stage of development change from the diageotropic position and 

 become orthotropous, or in inflorescences which become positively geotropic after 

 fertilisation. In this way the fruits of Trifolium subterraneum and of Arachis 

 hypogaea become buried in the soil. In twining stems also a change of tone has 

 been noted ; while young they do not twine. 



Geotropism as a Phenomenon of Irritability. The discoverer 

 of geotropism, KNIGHT, attempted to explain the geotropic move- 

 ments on purely mechanical lines ; this did not seem difficult, 

 especially for positively geotropic organs. He regarded them as 

 simply following the attractive force of gravity till a condition of 

 rest is attained. Later HOFMEISTER advanced similar views. The 

 correct assumption that we are concerned with complicated stimulus 

 mechanisms in which terrestrial gravity only plays the part of the 

 liberating factor depends on the work especially of DUTROCHET, 

 FRANK, and SACHS. Even the single fact that the root can carry out 

 its geotropic curvature against the resistance of mercury is sufficient 

 to call in question every purely mechanical explanation. 



Only in recent times has the attempt been made to determine 

 what is the primary effect of gravity in the plant ( 103 ). There is 

 no doubt that we are concerned with an effect of pressure ; the fact 

 that gravity can be replaced by centrifugal force is in favour of this. 

 This effect of pressure only comes into action in the case of ortho- 

 tropous organs in proportion as it acts at right angles to the longi- 

 tudinal axis, and thus in relation to the vertical component when 

 the organ is placed obliquely. Lastly, it is clear that the pressure 

 must act within the cells, and is in no way replaceable by external 

 influences. 



It is not known whether this pressure is determined by the entire cell-contents 

 and acts on the protoplasm as a whole, or whether special organs are concerned 

 in its production and reception. Various hypotheses on this question have been 

 advanced. F. NOLL first elaborated the idea that there must be some bodies in 

 the cells of greater specific gravity than the surrounding protoplasm, and capable, 

 under the influence of gravity, of exerting a one-sided pressure on the protoplasm ; 

 on this taking place the protoplasm directs the processes of growth in accordance 

 with the direction of the force of gravity. NEMEC and HABERLANDT then sug- 

 gested that these specifically heavier bodies (statoliths) might be found in certain 

 starch grains which show relatively rapid movements of falling in the cells. They 

 found such starch grains in the endodermis of the stem and in the cells of the 

 root -cap. They assume that the stimulus of gravity can only directly affect 

 portions of the plant provided with such starch grains, but that it may be con- 

 ducted from these points to others. As a matter of fact the attempt had 

 previously been made to show that only the tip of the root can receive the 

 gravitational stimulus. Even at the present time this question is not decided, 

 and not a few investigators assume that all cells though in various degrees are 

 geotropically sensitive. Thus fungi, in which statoliths are not found, are 

 geotropic. The hypothesis of NfeMEC and HABERLANDT, though there is much 

 in its favour, is not fully established. HABERLANDT himself states that in certain 



