No. 633] SHORTER ARTICLES AXD DISCUSSION 363 



branches and the characteristic form and type of growth of a 

 plant are thus determined. It is a species characteristic. 



An analysis of the factors which retard the growth of lateral 

 buds can best be made on plants with only a few buds, and an 

 excellent discussion of the problem has been given by Mc( 'allum.'-' 

 Mi-Callum worked with the scarlet runner bean, Phascolus mitlfi- 

 florus. The cotyledons of this bean remain at the surface of the 

 ground and the buds in their axils never develop unless the 

 growing stem is injured or removed. Then they invariably de- 

 velop and form shoots. No amount of wounding short of re- 

 moval of the growing tip will cause these buds to grow. They 

 never grow if the terminal bud is present, no matter how much 

 food or water is available with optimum light and temperature 

 conditions, and they always start to crow if the terminal bud is 

 removed and at the same time the plant is starved by cutting off 

 cotyledons and root systems or is practically dehydrated by plac- 

 ing it in a very dry atmosphere. 



To describe the phenomenon we say that the tip inhibits the 

 growth of buds below it. Only a growing tip has this inhibitive 

 action, for McCallum showed that if the tip is kept in a hydrogen 

 atmosphere, which prevents its growth, the cotyledonary buds 

 begin to grow. Later if the tip is removed from the hydrogen 

 to air, it also will again grow. 



The inhibitory influence of a growing stem tip on latent buds 

 is exerted only downward. A growing root exerts an inhibitory 

 influence on the development of roots above it and this influence 

 passes upward along the stem. If the roots of a bean plant are 

 removed, new roots will develop along the stem wherever there is 

 most moisture. The new roots do not come from root buds as the 

 new shoots come from shoot buds, but they arise from unformed 

 regions of the stem. The inhibitory influence on root formation 

 which passes upward moves along the vascular bundles and is 

 restricted to that section of the stem immediately above in line 

 with the point of injury to the root. This, if the stem of a bean 



itory action of the main roots will be cut off by the notch and 

 secondary roots can now form in this moist region only on the 

 side above the notch (see Fig. 10, p. 116 of McCallum 's paper). 



The inhibitory action of a growing tip in the bean on buds 



2 McCallum, Bot. Gas., XL, 97 and 241, 1905. 



