Growth, Auxin, and Tropisms in Decapitated Avena Coleoptiles.

With all the work on auxin appearing in the last years it is remarkable that not more investigations are concerned with the actual role of the auxins inside the plant. Barring some early opposition, the original statement "Without auxin no growth" (12) has stood unchallenged for many years. Probably this was so because all test methods were based on the principle of proportionality between auxin applied and its resultant effect. When BONNER and THIMANN (2) established that there was also direct proportionality between auxin used up and growth, the paramount importance of auxin in the growth process seemed firmly established. In later years, however, doubts have been expressed about the necessity of auxin for the growth process in general. AVERY and LARUE (1) concluded "that growth hormone is not a necessity for elongation of the coleoptile. " The confusion was due to the fact that auxin is present inside the plant in different states. This was pointed out especially by WVENT and THIMANN (15), and WENT (13), who stressed the point that it is essential to distinguish between auxin which can be collected by diffusion (diffusible or free auxin) and the auxin which is obtained by extraction. It is evident that only that part of the auxin which combines in some way inside the cells can be responsible for growth, whereas correlation phenomena such as tropisms can be brought about only by a correlation carrier which moves freely inside the tissues. Since no attempt has been made to correlate for one experimental object the relations between diffusible auxin, extractable auxin, growth rates, and tropisms the following paper describes experiments in which this was done for the Avena coleoptile. This is an especially favorable object for such studies since its growth rate and tropistic behavior can be radically changed by simple decapitation. The relation between auxin production and growth and tropisms was worked out by DOLK (3). After decapitation the growth rate of the Avena coleoptile falls off until two and one-half hours later when the growth rate suddenly rises to a value somewhat below its original rate. Geotropic and phototropic sensitivity show the same general dependence upon decapitation. Whereas tropistic sensitivity disappears immediately after cutting off the tip, it suddenly reappears two and one-half hours later. This behavior can be correlated with the auxin production in a coleoptile. Decapitation removes the production center of auxin, but two and one-half hours after removal of the tip, the cells immediately below the cut surface 236