GROWTH MOVEMENTS 1085 



Mcs I found a positive curvature again, i.e., toward the 100,000 mcs." 

 It appears that the retardation in growth obtained by Dillewijn with 

 an ilhimination of 800 mcs and the acceleration with 80,000 Mcs was the 

 result of a smaller and larger formation of growth-promoting substances, 

 respectively. 



Went concludes that, "The influence of light on a coleoptile of Avena 

 therefore appears in this investigation as in the former to be twofold: in 

 the first place it has an effect on the formation of growth regulators in 

 the tip, and secondly it temporarily diminishes the transport rate of the 

 growth regulators." 



The influence of these growth-promoting substances was further 

 studied by Uijldert (41). The removal of the flower buds of Bellis 

 perennis greatly retards the growth of the flower stalks. One experiment 

 will serve to illustrate the effect of the growth substance from Avena on 

 these flower stalks. In each of the following cases six flower buds were 

 cut off and the growth of the stalks noted after 24 hr. 



A. Flower buds replaced: growth, 1.43 mm. 



B. Agar with 2000 tip-minute growth substance per flower stalk (one "tip- 

 minute" is the amount of growth-promoting substances which diffuse out of 

 the coleoptile tip into an agar disk in 1 min. A value of 600 tip-minutes may 

 mean, therefore, the amount diffusing out of 6 tips in 100 min. or out of 12 tips 

 in 50 min.): growth, 1.93 mm. 



C. Pure agar without growth substance: growth, 0.40 mm. 



D. Control in which buds were simply cut off: growth, 0.63 mm. 



From the results found by Uijldert and later work it appears that the 

 growth-promoting substances of one plant may influence the growth of 

 another type of plant. These substances occur in such minute quanti- 

 ties that they are more or less comparable to animal hormones. They 

 occur in the coleoptiles of grasses, in various fungi, yeasts, and bacteria. 



Recently Dolk and Thimann (20) have attempted to determine the 

 chemical nature of these substances. Because of the difficulty of extract- 

 ing them from the tips of coleoptiles, they secured them from the fungus 

 Rhizopus suinus. The activity of their material was tested on coleoptiles 

 of Avena, as described by Went (44). Briefly, the method consists of 

 impregnating small blocks of agar and placing them on the sides of 

 decapitated coleoptiles and noting the curvatures after a given time 

 interval. These authors state that, "In order to be able to bring all the 

 data on to a quantitative basis, some kind of a unit system had to be 

 introduced. As unit was chosen that quantity of growth substance which 

 has to be present in 1 cc. of solution to give, after mixing with 1 cc. of 

 agar, an angle of 1 deg. The total number of units per cubic centimeter 

 of solution is then found by multiplying the angle measured by the 

 dilution in which the test was carried out. In the experiments below, 

 the first figure gives always the dilution, the second the angle actually 

 measured. The actual amount of material in the block applied to the 



