MOVEMENTS OF IRRITATION. 471 



at least that part of it can still be eliminated which is due to a 

 difference in the expansion under osmotic pressure of the cells of 

 the convex and concave sides of the structure, and which has not 

 yet been fixed by growth. 1 



1 Numerous references to the literature respecting heliotropism will be found 

 on pp. 303 and 304 of my Lehrbuch der PJlanzenphysioloyie, Breslau, 1883. 

 Many valuable details are given especially by Sachs and Wiesner, Denkschr. d. 

 Akad. d. Wiss. zu Wien, Bd. 39 and 43. 



179. Heliotropism (continued). 



To study more exactly many of the phenomena associated with 

 heliotropic nutations, w^e make the following experiments. We 

 arrange our clinostat in a dark room, in such a way that its shaft 

 is parallel to the rays of light entering by an opening in the 

 window shutters. A flower-pot, in which a Phaseolus plant raised 

 in the dark is growing, is fixed on the clinostat in the manner 

 described on p. 456. The light rays must travel in a direction at 

 right angles to the plane of nutation of the epicotyl, and this 

 latter must still be in active growth, and is marked off into 5 mm. 

 zones by means of ink-marks placed on the flanks towards and 

 away from the light. If now we set tne clinostat in movement, 

 pure heliotropic nutations are exhibited. Geotropic nutations are 

 eliminated, and moreover the spontaneous nutation of the epicotyl 

 does not interfere. The heliotropic nutation does not of course 

 appear at once, but after a little time. When it has become very 

 pronounced we stop the experiment, ascertain by means of the 

 cyclometer (a card marked with concentric circles of radii 1, 1*5, 

 2, etc. cm.) the radius of the curvature and determine the incre- 

 ments of growth. We find that the convex side of the epicotyl 

 has grown much more than the concave side of it. Only the still 

 growing parts of the epicotyl are capable of executing heliotropic 

 curvatures. If the experiment is continued long enough, all the 

 zones of the epicotyl which were in a state of elongation will take 

 part in the nutation in succession. 1 



It was emphasized in 169 that the s warmers of Algae are 

 repelled by intense light, while they are attracted towards 

 more feeble light. Hence it may be supposed that even the 

 organs of higher plants are attuned to light of a certain 

 definite intensity. Very thorough research, it is true, is still 



