LECTURE XVIII. 

 IRRITABILITY OF GROWING ORGANS (Continued]. 



Gravity. In a previous lecture (p. 409) some account was 

 given of the action of gravity in modifying the rate of growth. 

 We now turn to the consideration of the influence of gravity in 

 determining the normal direction of growth of growing organs 

 and of the phenomena which present themselves when a 

 growing organ is placed in a position other than that which it 

 normally occupies with respect to the line of action of gravity, 

 phenomena which are collectively designated by the term 

 Geotropism, a term which we owe to Frank. 



It must be premised that the effects produced by gravity 

 are of two kinds, namely, those which are due to the mere 

 weight of the parts, and those which are due to the stimulating 

 action of gravity. It is only the latter which are referred to 

 in using the word geotropism. It is of great importance that 

 these two sets of effects should be clearly distinguished from 

 each other, especially when we remember that Hofmeister 

 ascribed geotropic curvature in certain cases to the bending of 

 the organ under its own weight. Frank, however, demonstrated 

 that this view was entirely erroneous. It will be worth while, 

 in view of its importance, to illustrate this point rather fully. 

 Pinot, Mulder, and Payen found, for example, that the primary 

 root of a seedling will curve downwards into mercury, that 

 is, that it will curve downwards against a considerable re- 

 sistance, and Johnson observed that such a curvature was 

 performed against a weight of ten grains. It requires in many 



