The Conduction of Geotropic Excitation in Roots. 
BY 
R. SNOW 
(Research Fellozv of Magdalen College , Oxford). 
With four Figures in the Text. 
C ASES have long been known in which stimulus striking on one part of 
a plant organ leads to a responsive movement in another part: we 
have to suppose that the excitation set up in the perceptive region is con¬ 
ducted along the plant organ to the region of response. Such conduction 
seems to offer special opportunities for investigating what is the nature of the 
excitation conducted. But further interest attaches to those cases where the 
responsive curvature is carried out in a direction determined by the direction 
of the stimulus. For in these the intercalated phase of conduction seems 
to make it possible to examine in what way the direction of stimulus thus 
determines the direction of response. 
Amongst the best-known plant organs which show such conduction are 
roots and the so-called cotyledon of grass seedlings. In these, as shown 
by Ciesielski for the former and Rothert ( 1896 ) for the latter, excitation 
can be conducted back from the tip to produce a curvature in the elongat¬ 
ing region. 
A great advance was made when it was discovered by Boysen-Jensen 
( 1913 ) that in the case of the etiolated cotyledon of the Oat, Avena sativa , 
such conduction can take place through a layer of gelatine. For if the tip 
is cut off and stuck on again with gelatine, and then excited by stimulus 
of light, while the lower zones are kept darkened, responsive curvature 
towards the light will follow in the darkened lower region. This has been 
confirmed for various other grasses by Paal ( 1918 ), and further shown to 
hold for traumatic stimulus in many other seedlings (Stark, 1921 ). 
It therefore occurred to the writer to investigate by similar methods 
the old problem of the conduction, from root-tip to elongating region, of the 
excitation set up in the tip by the stimulus of gravity. 
(Annals of Botany, Vol. XXXVII. No. CXLV. January, 1923.] 
