198 PLANT PHYSIOLOGY 



as can be done by a centrifugal machine, the rate of growth 

 may be changed. But when the position of a plant is con- 

 stantly changed or is changed at frequent and regular inter- 

 vals, as by a regular or by an intermittent clinostat, the 

 rate of growth does not seem to be materially affected.* 



Gravity is one of the most important influences determin- 

 ing the direction of growth. It affects direction and kind 

 much, the rate of growth little. Its action in directing 

 growth is called geotropism. Those organs which grow 

 toward the source of gravitation, downward, are said to be 

 positively geotropic. Roots and rhizoids are positively 

 geotropic organs. Stems which grow in the opposite direc- 

 tion, upward, are called negatively geotropic. Other organs, 

 such as branches, which grow horizontally, are said to be 

 diageotropic, while obliquely growing organs like lateral 

 roots are called plageotropic. For reasons of convenience, 

 roots have long been the favorite objects for the study of 

 the effects of gravitation. Such horizontal organs as leaves 

 owe their position quite as much to the influence of light as 

 to gravitation. Though we may say that gravitation is the 

 chief force directing the growth of stems and roots, it must 

 be remembered that it is only the chief of several or many. 

 The position which any part or organ finally assumes repre- 

 sents the combined influence of all of those forces to which 

 it is sensitive and which act upon it. 



It may be stated as a general rule that the stems and 

 roots of higher plants, and the corresponding parts of 

 many lower plants, tend to grow in opposite directions. 

 Each organism begins as a single cell. Upon this one cell 

 all the influences which affect the new plant are converged. 

 The directions of growth and of division of this one cell are 

 the result of these influences. In consequence of the first as 

 well as of subsequent divisions, the different cells of the 

 embryo are differently placed, some opposite to others. 

 From this oppositeness in position there follows an oppo- 

 siteness in behavior, which expresses a difference in the cells 

 and organs themselves. It is easy and natural to suspect 



* For a discussion of this topic see Pfeffer's Pflanzenphysiologie, 2te 

 Aufl., Bd. II., § 29, 1901. 



