196 PLANT PHYSIOLOGY 



the morbid growth begins and ends, and with the differences 

 in the activities of the galled leaves from those which are 

 normal. 



The formation of wood and of its different kinds and 

 elements in a season is affected by aU the vital activities 

 of the plant and all the external influences which bear 

 upon it. The character of the wood is not the result of 

 any one set of factors. At the same time that we must 

 constantly recognize that the living plant is sensitive to a 

 great many influences and that it responds to these, we may 

 distinguish in the complex of influences some which are more 

 effective than others. We may therefore accept, at least un- 

 til a better one is advanced, this hypothesis : the two kinds 

 of wood in the year's growth are formed in their different 

 ways in response to the different demands, or stimuli, 

 brought to bear upon the cambium and its young deriva- 

 tives; the "spring wood," composed of large elements, essen- 

 tially for the conduction of liquids; the "autumn wood," 

 composed of small and thick-walled elements, essentially for 

 mechanical strength; and between these two, the wood 

 which is both "spring" and "autumn" in character, formed 

 when there is still need of more conducting tissues and when 

 the need of strengthening tissues is already beginning. So 

 we have the adaptation of the wood to the different needs 

 of the plant in different parts of the growing season, the 

 adaptation being accomplished through the irritability of 

 the growing and differentiating cells. 



INFLUENCE OF GRAVITATION. 



So far we have studied mainly the effects of evidently 

 mechanical influences upon the living organism. It is, how- 

 ever, sensitive to many other influences, some of them quite 

 as important. Of these only one is constantly and uni- 

 formly operative — ^the force of gravitation. The plant may 

 change, by growth and movement, the relation of its parts 

 to the force, but the amount of force acting upon the plant 

 continues the same. The other influences are variable, 

 periodic, or occasional. The mechanical influences so far 



