CH. Vl] TEMPERATURE. 157 



ness if the root is illuminated horizontally by means of an 

 oblique mirror. Bean roots are so slightly heliotropic 1 

 that there is no objection to a lateral light. Other roots, 

 e.g. the apheliotropic radicles of Sinapis, can only be 

 observed when rotated on a vertical axis. Spontaneous 

 curvatures do however occur in the bean root, and may 

 spoil the experiment : that known as Sachs' curvature 

 which occurs in the plane of the cotyledons is especially 

 troublesome 2 . Askenasy 3 uses maize roots for his growth 

 experiments; he notes that the circumnutation of the 

 roots is in some cases very considerable. 



Read the position of the tip of the root after 10 or 

 15 minutes ; and again after a like interval of time ; if the 

 rate of growth is fairly uniform for the two periods, the 

 temperature of the water may be at once raised, if not 

 further readings must be taken. The water may con- 

 veniently be siphoned out and replaced by warmer water. 

 It is best to use water 10 or 12 C. warmer than that in the 

 vessel. The growth of the root will be at once accelerated, 

 becoming nearly three times as quick. It will again fall 

 as the water sinks to the temperature of the room. 



(184) Temperature : microscopic method. 



Repeat exp. 183, substituting water at 3 or 4 C. for 

 the water in which the observations were begun. Note 

 the sudden and serious check to growth. 



1 According to Kohl (Die Mechanik der Eeizkrilmmungen) bean seedlings 

 grown on the klinostat show distinct apheliotropism. 



2 Poiver of Movement in Plants, p. 91. 



3 Deutsche botan. Gesellsch. 1890. 



