810 D. P. Gowing 



in old rubber trees which is being conducted at the Rubber Research 

 Institute in Malaya. The new clones of rubber trees in Malaya are 

 many times as productive as the unselected trees planted 30 to 40 years 

 ago. There is a great need to replace the old trees as rapidly as pos- 

 sible, but they cannot all be replaced at once because then there would 

 be no rubber on the estate. It has been found that if esters of substi- 

 tuted phenoxyacetic acids are applied below the tapping panel in an 

 oil vehicle, the rubber flow is appreciably increased for as much as 

 six months. It is relevant to the discussion of the Avena test that 

 2,4,5-T causes the greatest stimulation, followed by 2,4-D. Substitution 

 in the 6 position, dimethyl substitution, or monochloro substitution, 

 all greatly decrease the activity. The effects of other growth regulators 

 are now being explored. 



Dr. Fawcett: How do 3,4-dichloro- and 2-methyl-4-chlorophenoxy- 

 acetic acids compare with 2,4-D for the promotion of flowering? 



Dr. Gowing: In connection with Prof. Blackman's comments, 4- 

 chlorophenoxyacetic acid, 2,4-D, and 2,4,5-T show about the same 

 order of effectiveness in induction of flowering in pineapple as in 

 stimulating the flow of latex. The 2,4,5-T is the most active of the 

 group. 



Dr. Fawcett asked about the 2-methyl-4-chloro- and the 3-methyl-4- 

 chlorophenoxyacetic acids. The 3-chloro-4-methyl compound has 249 

 per cent of the activity of 1-naphthaleneacetic acid in the split pea- 

 stem curvature test but doesn't induce flowering in pineapple. How- 

 ever, the 3-methyl-4-chloro compoiuid does induce flowering. Similarly, 

 the 2-chloro-4-methyl compound has no activity in flower induction but 

 has considerable activity in the split pea-stem curvature test, and the 

 2-methyl-4-chloro compound (MCP) is active in both tests. MCP is 

 active in flower induction at about the same level as 2,4-D (0.6 mg. per 

 plant for 50 per cent or more induction). The 3,4-dichloro compound 

 induces flowering in the pineapple at about 3 mg. per plant for 50 

 per cent or more effectiveness. Consequently, it is excluded from com- 

 mercial consideration as an herbicide in pineapple plantings. 



Dr. Sachs: In view of the multiplicity of the responses observed by 

 Dr. Gowing, such as effects on flower initiation, herbicidal effects, etc., 

 I am wondering about the mechanism and site of action of these sub- 

 stances. Is there any universal point of view that can be developed? 

 Will it be one Kingdom of Heaven or seven? I think there has been 

 a general impression that the primary site of action of most growth 

 regulators is in the cell walls, probably by causing them to become 

 soft and thereby letting them extend. I don't hold to this view, or 

 at least I don't see how one can jump from softening a cell wall to 

 getting a nucleus to divide to getting flowers to be initiated. 



