MEASUREMENT OF THE RESPONSE OF LEMNA TO GROWTH PROMOTING SUBSTANCES

THE RESPONSE of Lemna to growth promoting substances of one kind or another is a subject which has occupied the attention of numerous investigators since 1914. The series of experiments to be described were performed in order to ascertain what effect, if any, treatment with small amounts of 3-indoleacetic acid, 3-indolebutyric acid, 1-naphthaleneacetic acid, or vitamin B1 would have upon the growth of Lemna minor L. since these substances are recognized as possessing growth stimulatory properties. The relative rate of increase in frond number and increase in total frond area were employed as criteria for measuring the response to these substances. Treatment involved either addition of the test material to the nutrient culture solution or application by means of lanolin paste to the plants themselves. According to Clark (1925), increase in frond number is exponential, a graph plotting the logarithm of frond number against time in days gives a straight line, the slope of which is a measure of the rate of increase in the number of fronds. Throughout each experiment, therefore, frond numbers were recorded daily. From assembled data regression coefficients (slopes) of lines of closest fit were calculated by the method of least squares. By applving the analysis for variance, significant differences between the calculated regression coefficients of different treatments were ascertained. Ashby, Bolas, and Henderson (1928) measured the area of Lemna fronds by floating them on water contained in a shallow dish illuminated from below. The silhouetted image was magnified by a lens in a fixed position between this dish and a ground glass screen. Upon this screen tracings were made at a known constant magnification. Ashby and Oxley (1935) replaced the ground glass screen with a photoelectric cell to which a sensitive galvanometer was attached. Area differences were expressed in terms of the extent of the deflection of the galvanometer needle. To obviate certain disadvantages inherent in these methods a modification was devised. An inexpensive 35 mm. camera was dismantled and the lens extended so as to give a focussing range of approximately 8 inches. The reassembled camera was fastened with the lens projecting through a hole in the top of a box painted flat black inside and out. To the rack and pinion on the stand of a binocular microscope a ground glass topped illuminator was attached which contained a pilot light and a photoflood light. Stand and illuminator were fastened in the box with the camera in such a position that the portion of the illuminator directly beneath the lens was the most intensely lighted. The focal distance was established and marked by a fine-pointed needle 1 Received for publication July 6, 1940. This work was done at the University of Maine under the direction of Dr. G. P. Steinbauer. projecting from the side of the box. A 250 ml. culture beaker placed on the illuminator was raised or lowered until the level of the liquid was aligned with this needle. Beside each culture beaker a small square of celluloid with the corresponding number on it was supported on a piece of glass at the correct focal level. The photoflood lamp was turned on for a brief interval and a photograph of the plants on the surface of the solution with the culture number beside them was made.