GROWTH SUBSTANCES FOR GEOTROPISM 197 



pendant over erect fruit in the Fi generation. Van Overbeek 

 (1936a) studied geotropic response in a variety of maize commonly 

 referred to as "lazy." He reports that when grown in darkness 

 the young plants up to 5 or 6 days old are negatively geotropic, 

 as are normal seedlings; when grown in light, the plants are 

 ageotropic. Brain (1933) observed a seasonal variation in the 

 gravitational irritability of Lupinus arhoreus and L. polyphyllus, 

 while L. alhus appeared to be equally responsive throughout the 

 year. The first two species were termed "physiologically 

 zygomorphic" because their sensitivity to gravity was greater 

 for the cotyledonary than for the intercotyledonary plane, while 

 the last was equally sensitive for both planes of the hypocotyl. 

 These and other variations between closely related plants 

 seem to indicate clearly that certain tropic properties may be 

 inherited. 



Geotropic Response in Nodes. — It is well-known that in many 

 plants (particularly in the grasses) portions of the axis in the 

 region of the nodes remain potentially embryonic long after the 

 rest of the internodal parts have differentiated into mature 

 tissues. When such plants are displaced from their normal 

 position with respect to gravity, negatively geotropic curvatures 

 occur. 



The geotropic response of mature grass nodes was investigated 

 by Sachs (1872) and DeVries (1880). When grass stems were 

 placed in a horizontal position, growth in length of the nodes was 

 renewed on the under side, while the upper side became com- 

 pressed. Two effects of gravity should be distinguished in this 

 case: (1) the resumption of growth and (2) the unilateral distribu- 

 tion of growth, Elfving (1884) made the noteworthy discovery 

 that growth was resumed under the unlocalized action of gravity, 

 attained when the plant was rotated on a clinostat. 



As has been mentioned in the chapter on normal growth, 

 Schmitz (1933) demonstrated the presence of growth substance 

 in young growing internodes. In spite of possessing growth 

 substance, the young internodes do not curve geotropieally, nor 

 can accumulation of growth substance on one side be demon- 

 strated. Young nodes also contain growth substance, but the 

 mature ones do not. However, when a plant with mature nodes 

 is rotated on a clinostat, formation of growth substance occurs 

 apparently as a result of the unlocalized action of gravity. 



