CHAPTER XXXVI 



THE RESPONSIVE CURVATURES CAUSED BY GRAVITV. 

 NEGATIVE GEOTROPISM 



Statement of the problem of apogeotropic response— Mode in which stimuhition 

 is brought about : radial-pressure theory, and theory of statoliths — Mechanics 

 of responsive movement — Experiment demonstrating responsive curvature as 

 brought about by unilateral pressure of particles— Record of curvature induced 

 by gravitation— Record of different rates of curvature when specimen is held 

 at angles of 45° and 135° co the vertical— Determination of the true character 

 of apogeotropic response — Responsive curvature of acellular organs— Curvature 

 of grass haulm under gravity— Growth of grass haulm on a klinostat. 



It is well known that growing organs exhibit certain 

 directive movements under the influence of gravitation. 

 Horizontally laid shoots, for example, bend upwards, or 

 against the direction of gravity ; while roots react in pre- 

 cisely the opposite way — that is to say, they bend so as to 

 lie in the direction of the force of gravity. In the case of 

 the various forms of stimulation hitherto studied, the action 

 of the plant is well defined and intelligible, consisting of con- 

 cavity of the excited side, in response to a stimulus which is 

 clearly understood. But in the case of geotropism the mode 

 in which stimulation is brought about is not quite evident, and 

 we find, moreover, two directly opposed effects brought about 

 by apparently the same force of gravity : in the root, as 

 already said, a positive movement, that is to say a movement 

 in the direction of gravity ; and in the shoot a negative move- 

 ment, away from the direction of gravity. 



The seeming impossibility of explaining effects so divergent 

 as due to a single common cause, has led to the modern 

 idea that these responsive movements are ' executed at the 

 suggestion of changes in the environment, not as the direct 



