THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. 275 



keystone of our knowledge already gained and that for which 

 we are striving. A similar difficulty in finding a causal explana- 

 tion is met with in negative and positive geotropism, which we 

 shall now discuss. 



C. INFLUENCE OF GKAVITY. 



The plant kingdom is subject to the continuous influence of 

 gravity. Light and temperature have their variations in the 

 change of day and night and in the seasons of the year. Gravity 

 is a constant factor and has a great influence on plant-life. 



If a growing primary root is placed horizontally, it will at 

 once begin to curve downward at the growing part, due to the 

 more active growth of the upper side. A portion of the stem 

 (growing shoot) or the base of the leaf-sheath of a grass-inter- 

 node * placed horizontally will curve upward because of the 

 more rapid growth of the lower side. This is the so-called posi- 

 tive and negative geotropism. 



It can readily be shown that it is gravity which causes the 

 downward growth of roots (positive geotropism) and the up- 

 ward growth of stems (negative geotropism). If the centrifugal 

 machine is employed to counteract the earth's gravity, it is 

 found that the root grows outward and the stem grows in the 

 opposite direction toward the centre of rotation. 



Secondary roots, branches, and leaves are controlled by other 

 factors, since they grow in a diagonal or horizontal direction 

 under the influence of gravity. According to SACHS, the latter 

 organs are said to be plagiotropic, in distinction to the orthotropic 

 stem and primary root. 2 SACHS emphasizes the fact that dor- 

 si ventral organs are plagiotropic ; this one would expect when 

 considered from the standpoint of advantageous adaptability. 

 The horizontal position is certainly more suitable for organs 

 with one side adapted to light and the other to comparative 

 darkness. According to the same author, all orthotropic organs 

 have a radial structure (see p. 166) which corresponds to the 



1 Concerning this behavior of leaf -sheaths and nodes of Gramincce see Part II, 

 B, Function V. 



2 Sachs also applies these terms to a similar behavior of plants in response to 

 light-effects. 



