Some effects of organic growth-promoting substances (auximones) on the growth of Lemna minor in mineral culture solutions

In a previous communication experiments were described which indicated what when peat is incubated with a mixed culture of aerobic soil organisms for about fourteen days at a temperature of 26°C., a rapid decomposition of the organic matter takes place, with the formation in the "bacterised" peat of certain organic growth-promoting substances or "auximones", the addition of which in very small amounts to wheat seedlings growing in water culture causes a marked increase in growth. It was desirable to repeat these experiments with plants in which any variation in growth could be readily and more accurately estimated than in wheat seedlings. There was a difficulty at first in selecting a suitable plant for experiment. The objections to using the seedlings of land plants are: the difficulty of accurate weighings at regular intervals; the fact that a water culture solution is not the natural habitat for a land plant; the possibility that such seedlings may contain a supply of organic growth-promoting substances produced from the endosperm during germination. Water plants, on the other hand, are usually considered unsuitable for water culture experiments because they will not grow for any length of time in pure mineral culture solutions. Darwin and Acton state that “water plants cannot generally be recommended for accurate experiments extending over any considerable time, as we have found it much more difficult to grow them satisfactorily in culture solutions than to grow ordinary plants with the roots immersed.” They say, however, “we have found Lemna minor useful for purposes of demonstration. They grow rapidly, and their increase being principally in one plane is easily noticed at a glance. Moreover a rough numerical estimate of the amount of increase in a given time can be made by counting the fronds.”