STIMULATION OF GROWTH BY CROWDING 159 



lated the growth of Lemna and other water plants to a marked de- 

 gree. Bottomley gave the name "auximones" to these substances 

 which were effective in promoting growth for green plants. It soon 

 became apparent that the green plant Lemna can grow and multiply 

 for indefinite periods in a purely inorganic medium (Clark, 1924, 

 1926; Ashby, 1929), and Wolfe (1926) was led to the point of view 

 that Bottomley's theory of the need of growth-promoting substances 

 by green plants was completely refuted. Ashby made a more com- 

 plete analysis of the problem (1929a) and has demonstrated that 

 small amounts of organic substance obtained from fresh horse dung 

 will increase the rate of growth of Lemna if present in only 0.2 parts 

 per million and that the growth-rate is little affected by additions of 

 this material beyond 2.0 parts per million. 



The duckweed which Ashby used in these experiments had been 

 growing for 6 months on a purely inorganic medium made up in 

 glass-distilled water. Environmental conditions such as light, tem- 

 perature, and pH were adequately controlled. The mean frond 

 weight remained the same in control and experimental solutions; 

 the mean frond number increased 42 per cent; the area of the fronds, 

 33 per cent; and there were roughly 80 per cent more chloroplasts in 

 the cells of fronds treated with 0.002-0.02 grams per liter of dry ex- 

 tract, as compared with untreated fronds. 



The addition of 0.2 parts per million of organic matter to a solu- 

 tion containing already 1,210 parts per million of mineral matter 

 will significantly increase the growth of Lemna. The power of the 

 extract is not affected by autoclaving; hence the effect is not due to 

 an enzyme. The ash constituent of the extract does not increase the 

 growth-rate, and increasing the nitrogen content by adding 0.003 

 grams per Hter of KNO3 did not affect the growth, while adding one- 

 hundredth of this amount of nitrogen as organic matter did signifi- 

 cantly increase the rate of growth. It seems clearly estabhshed by 

 this work that,, while Bottomley's "auximones" are not essential for 

 plant growth, the addition of extremely minute amounts of organic 

 material produces this effect by acting as a catalyzer. 



The work with vitamins, tissue extracts, bios, and auximones in- 

 dicates clearly that the presence of very small amounts of organic 



