310 



Gene Action 



during development, the growing tip of the stem curves upward 

 and the new growth is upright. Such growth is said to be nega- 

 tively geotropic because it is against the direction of gravity. 

 Lazy plants, however, when placed at the same angles fail to 

 bend upward and continue to grow in the direction in which 

 they are placed. Numerous studies during the 1930's have shown 



r\ 



u 



\ I 



) 



Concentration 

 of auxin 



Normal maize horizontal 



3 



J 



No concentration 



No greater growth 



Normal 



maize 



vertical 



"Lazy maize horizontal 



Fig. 86. Diagram to indicate the action of auxin in maize stems. When 

 a normal branch is placed on its side, the hormone becomes concentrated 

 on the lower side, causing this side to grow more rapidly than the upper 

 and thus causing the stem to gi^ow upwards. In the homozygous recessive 

 "lazy" type, there is no movement of the hormone to the lower side; 

 there is no greater growth there, and the stem does not curve upwards. 

 (Based on the work of van Overbeek.) 



that growth in plants can be stimulated by a hormone known as 

 *'auxin." When a normal maize plant is placed on its side, this 

 auxin diffuses because of gravity to the lower part of the stem. 

 Growth is then stimulated on the lower side, and the lower side 

 grows more rapidly than the upper. This greater growth of 

 the lower side makes that side longer than the upper and the 

 stem bends upward (Fig. 86). In lazy maize, there is no such 

 redistribution of the auxin, the low^er side does not grow more 

 rapidly than the upper, and the stem continues to grow straight 

 and does not bend upward. The la gene apparently interferes 

 with the distribution of auxin which occurs in La plants. This 

 is another example of a phenotypic effect produced by a gene 

 by the action of hormones. 



