198 GROWTH HORMONES IN PLANTS 



Growth substance is formed anew under the unilateral action of 

 gravity, but it is found only on the under side. When geotropi- 

 cally stimulated nodes were excised, split lengthwise, and placed 

 unilaterally upon decapitated Avena coleoptiles, a curvature 

 resulted from use of the under halves only. These experiments 

 were carried out with nodes of Triticum, Secale, Lolium, Holcus, 

 and Setaria. In an experiment using the under halves from 

 40 test plants, there were 27 negative, 10 straight, and 3 positive 

 curvatures ; using the upper halves of nodes from 26 plants, there 

 were 3 negative, 20 straight, and 3 positive responses, A clear 

 difference exists between the growth-substance content of the 

 upper and lower halves of geotropically stimulated nodes. 



There are two possibilities concerning the origin of growth 

 substance as a result of the unilateral effect of gravity. Either 

 the growth substance is formed exclusively in the cells of the 

 under half of the node, or else it is formed in all cells and is imme- 

 diately displaced to the under side as a result of gravity. Schmitz 

 deals with the first possibility in the following words: "Growth 

 substance is formed anew upon the under side of a node when it is 

 unilaterally stimulated by gravity, but it is formed in the whole 

 node when it is subjected to unlocalized stimulation on a clino- 

 stat." The second possibility of general formation followed by 

 unilateral displacement is the most probable. The growth and 

 curvature in halved nodes lend strong supporting evidence. As 

 deVries originally showed, the upper as well as the lower half of 

 nodes can curve geotropically which must mean that growth sub- 

 stance is formed in the upper half also, when it is in a horizontal 

 position. The fact that it cannot be demonstrated there in 

 intact nodes can be explained as due to displacement to the lower 

 side. 



While the nodes of grasses act independently in geotropic 

 curvature, in other plants, such as Tradescantia, there is some 

 relationship between the curvature of one node and the node 

 next above it. Kohl (1894) showed that the negatively geotropic 

 curvature of a node of Tradescantia could be suppressed by 

 removal of the node above. He concluded that the stimulus is 

 received in the upper node and is transmitted from there to the 

 lower node where the reaction occurs. Miehe (1902) showed that 

 this view was not correct, for, if the internode between the two 

 nodes was bent so that the upper node was vertical and the lower 



