l6 PLANT GROWTH SUBSTANCES 



phenomena. For example, van Overbeek et al. (47) have shown that 

 the geotropic response of horizontally placed sugar cane is due to the 

 formation of free auxin at the expense of the bound form in the lower 

 side of the node. 



In a search for growth factors other than auxins the effect on the 

 seedling of removal of the endosperm was studied. Even after supplying 

 sugars and mineral salts the growth of young seedlings is then greatly 

 retarded. In experiments on pea seedlings it was shown that when a 

 cotyledon was placed in the nutrient medium together with the embryo 

 considerably better growth is obtained. Since it was known that several 

 vitamins were present in the endosperm these were tried, and it was found 

 that biotin, vitamin Bi, ascorbic acid, and even the animal hormone, 

 estrone, caused increased growth of the seedlings over the controls. The 

 addition of thiamin was characterized by a greatly increased root system 

 (26). This effect is also observed in isolated roots, and Bonner (5) has 

 shown that vitamin Bi is a major factor in the development of the root, 

 and that its function is supplemented by pyridoxine and probably 

 nicotinic and pantothenic acids (6). The material diffusing from the 

 cotyledons is apparently rich in growth substances, a conclusion sub- 

 stantiated when pea diffusate was found to be active in promoting leaf 

 growth. Using the growth increase of circular discs of tobacco and radish 

 leaves as a test method, adenine was isolated and recognized as one of the 

 active agents. Embryonic pea leaves also showed increased growth under 

 the influence of adenine. For this tissue, therefore, adenine acts as a 

 typical plant hormone (3). Some investigators have found that the 

 auxins also affect leaf growth. Went (65) has given the name rhizocaline 

 to the complex of chemical factors other than auxin which are involved 

 in root formation. They are found in buds and cotyledons and are formed 

 by leaves in the presence of light. The nature of these substances is 

 unknown. The same is true of caulocalines, factors which influence stem 

 growth and are produced in roots, and the postulated flower hormones. 



We are better informed regarding the nature of a different type of 

 growth activator: the wound hormones. Upon injury of a plant the intact 

 cells surrounding the wound show a greatly increased rate of division. 

 Wiesner (68) suggested in 1892 that special substances are probably 

 produced by wounded cells. Haberlandt (20,21) found that the surface 

 of potato tuber responded to the application of phloem tissue and to 

 crushed cells or their extracts. Similar phenomena can be studied on 



