Scientific Results 



substance, as a relatively small number of cells were directly 

 affected. In many cases the active substance was that absorbed 

 from a plant-surface by a moist agar surface during a very 

 short time, a small block of the agar being then cut out and 

 applied to the cut surface of another (demonstration) plant, 

 which showed by a bending response that it re-absorbed 

 some of the substance present in the first plant. An Avena~ 

 Einheit (AE) or Avena unit (A.U.) has been devised. Full 

 details of this technique and of its numerous refinements will 

 be found in Boysen Jensen's book. 



It was soon realized that urine and the resources of bio- 

 logical and synthetic chemistry laboratories could provide 

 larger amounts of growth-promoting substances than those 

 yielded by a transverse section of a plant. The delicate 

 oat-coleoptile {Avena) test has remained in use for purely 

 physiological research, but for more practical purposes it has 

 been succeeded by grosser tests largely based either on root- 

 formation — often in highly unusual positions on the plant — 

 or else on the phenomenon known as epinasty} Epinasty is an 

 expression by a mature plant of a set of changes due to 

 unequal development of various parts, so that bending, and 

 sometimes twisting, develops. For example, if one side of a 

 stem or leaf receives more growth-promoting substance than 

 the other, the stem or leaf will bend or curl. The phenomenon 

 of epinasty is thus not different in principle from the simple 

 and measurable bending of a decapitated coleoptile of an oat 

 seedling, but visible epinastic changes in mature plants are 

 complicated by the presence of asymmetric structures of 

 various degrees of rigidity — such as the tapering "veins" in 

 the leaves. For this reason epinastic changes must be regarded 

 as gross qualitative manifestations. 



Growth Promotion and Growth Regulation: a Distinction 



The production of roots, or, to speak more accurately, 

 the initiation of root-formation, is that property of growth- 



^ Epinasty and its opposite, hyponasty, are together known as nastic 

 movements or responses. The original papers should be consulted for fuller 

 details, including a discussion of the hyponastic effects, epinasty being taken 

 as the type of both for the purpose of this introductory explanation. 



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