V. RELATION OF PLANTS TO GRAVITATION 



109. Nature of the Relation of Gravitation to Plants. All of 

 the agencies besides gravitation, which affect protoplasm, or irri- 

 tate it in any manner, consist of some form of chemical or physical 

 energy which may affect the molecular motions of living matter 

 directly. Gravitation is a force however, which has no direct ef- 

 fect upon protoplasm, although it causes well-defined reactions. 



The emergence of the primitive- ancestors of the higher plants 

 from aquatic habitats brought the vegetal organism into a new 

 set of conditions both for nutrition and distribution of reproduc- 

 tive bodies. As a result of these conditions, and also of the 

 fierce competition arising from the enormous multiplication of in- 

 dividuals, and consequent increase of the number of species, it 

 became highly advantageous for the plant to hold its shoot in an 

 upright position above the substratum, and to send the roots or 

 absorbing organs in the opposite direction. A sensory organiza- 

 tion for the maintenance of such positions finds only one constant 

 force by which it may be guided, which is gravitation. The irri- 

 tability to gravity which has been acquired by the plant is there- 

 fore one of association, and not one of direct causal adapta- 

 bility. 



Beside the principal reactions by which the body is placed 

 parallel to the line of action of gravitation, differentiations of this 

 form of irritability enable the branches and secondary organs, 

 leaves, etc., to assume a position at, or near, a right angle to the 

 line of action of gravity. 



Experimental tests of the action of gravity upon protoplasts 

 by means of centrifugal force show that abnormally intense grav- 

 itational forces merely cause mechanical disarrangement of the 

 organs of the cell according to their weight and density, and 

 since such experiences do not accrue to the plant, no adaptive re- 



7i 



