Ph)'siologie, 293 



transient depression, followed by revival of excitability; exposure 

 to light caused a transient depression, followed by an enhancement 

 of excitability; excessive lurgor induced a deminished response; 

 lowering of temperature induced a depression culminating in abo- 

 lition of response, the aftereffect of excessive cold was a persistent 

 depression; rising temperature enhanced excitability up to an Opti- 

 mum, above which a depression was induced. F. Cavers. 



Hiley, W. E., On the Value of Different Degrees of Cen- 

 trifugal Force as Geotropic Stimuli. (Ann. Bot. XXVII. 

 p. 719—758. 2 pls. 3 text-figs. 1913.) 



After a hislorical introduction the author disciisses the extent 

 to which the geotropic conditions of Stimulus on a centrifugal wheel 

 resemble those under the Stimulus of gravity acting on a horizontal 

 plant-member, and points out that the expression "centrifugal force" 

 is of doubtful accuracy as generally used in botanical literature to 

 denote the geotropic Stimulus obtained on a centrifugal wheel. On 

 such a wheel a plant-member is always tending to fly off at a tan- 

 gent but is prevented from doing so by being held at a constant 

 distance from the centre of rotation, hence it is subjected to a 

 centripetal acceleration. and it is this which causes the roots to 

 bend centrifugally — i. e. in the direction opposite to that of the 

 acceleration; thus the so-called "centrifugal force" might more pro- 

 perly be expressed in terms of this centripetal acceleration. On the 

 other hand, when we consider any sense organ the term "centri- 

 fugal force" acquires a new meaning; thus in terms of the statolith 

 theory, on the centrifuge the starch-grains move to the outer side 

 and actually do exert a centrifugal force on the outer walls of 

 he statocyte, and in this sense the term may be used without 

 violence to scientific accuracy. 



The author then describes an intermittent centrifuge , with notes 

 on working methods and sources of error, and gives the results of 

 his experiments with this instrument, which was designed so as to 

 give alternate short-timed exposures to gravity and a centrifugal 

 force in opposite directions. It was found that if a centrifugal force 

 C, working for a time T, be regularl}^ alternated with gravity (mg.) 

 working for a time t, then equilibrium is only established when 

 CT/mg. t^l; the actual value as deduced from the average of 18 

 successful experiments with Helianthtis radicles was 0.99 that for 

 6 experiments with Cucurbita radicles 0.97. The equation does not 

 hold good if the individual periods of exposure to gravity and 

 centrifugal force are long. When T -j- t was as much as 20 minutes, 

 CT/mg. t was greater than 1 for the point equilibrium. that is, the 

 centrifugal force had to be allow^ed to act for a longer time than 

 would have been expected from previous experiments. This is pro- 

 bably connected with the fact, discovered by Jost and Stoppel, 

 that radicles may respond negatively to prolonged exposure to cen- 

 trifugal forces. The presentation time for Helianthus radicles, deter- 

 mined in the usual way, was found to lie between 3 and 4.5 minutes 

 at 18°— 20°C.; on the intermittent centrifuge, Single exposures to 

 gravity mi^rbt be much longer than this without any failure of the 

 equation CT/mg. t=l. F. Cavers. 



Keeble, F., E. F. Armstrong and W. N. Jones. The Forma- 



