IRRITABILITY 197 



discussed may be said to control growth by limiting it. 

 The influences which we are about to study control growth 

 by directing it. Yet this distinction is suggestive rather 

 than exact, and must not be accepted without reserve. 



The action of gravitation may be considered from its 

 effects on the kind, rate, and direction of growth, and on 

 the position, of plants. Gravity exerts upon all objects a 

 pull toward the centre of the earth. This pull is propor- 

 tioned in intensity to the weight ( mass ) of each object, the 

 heavier the object the stronger the pull. The pull is resisted 

 more or less completely by the medium in which the objects 

 are. An object in a fluid is buoyed up with a force equal to 

 the weight of the fluid which it displaces. Thus the down- 

 ward pull of gravity upon a plant living submersed in fresh 

 water is resisted by a force equal to the weight of the water 

 which the plant displaces. This is 750-800 times greater 

 than the force which an equal volume of air would exert. 

 The average specific gravity of sea- water is 1.2. Hence a 

 plant living in sea- water is supported still more, by a force 

 1.2 times greater than that of an equal volume of fresh 

 water, and therefore 900-950 times greater than air. The 

 parts of a plant growing in a solid medium, the soil, are 

 completely supported. The soil will ordinarily support 

 much more than the weight of the plants growing in it. It 

 is evident, therefore, other things, being equal, that the 

 mechanical strength which the plant or plant-part must 

 develop is proportioned to the fraction of the force of 

 gravitation which is not balanced by the buoyancy or sup- 

 porting power of the medium in which it lives. The force of 

 gravity exerts by this means a direct influence upon the 

 kind of tissue which the plant forms, the kind of growth 

 which it makes. The force of gravity is one of the most 

 important factors in the complex which constitutes the 

 environment. 



The rate of growth in most plants seems to be tolerably 

 independent of gravity, other forces being more effective. 

 Parts which normally stand in one direction may grow at 

 a somewhat different rate when their position is changed. 

 Also, when gravity is opposed by an equal or greater force, 



