Summary: This study examined the effects of competition on the growth of Avena fatua, winter wheat and winter barley. Plants were sampled at frequent intervals from replacement series experiments at two contrasting sites in the U.K. A. fatua was much slower to establish than the two cereals, but thereafter exhibited a faster rate of growth. In monoculture, it took a considerable time for A. fatua to reach a size equal to that of the cereals, but by the end of the experiments it was the largest of the three species. The change‐over from cereal dominance in mixtures to A. fatua tiominance was rapid, and in three cases coincided with cereal flag leaf emergence. In the fourth case, it appeared to coincide with the art of canopy height extension. At one site the rder of competitiveness at anthesis was A. fatua= barley>wheat, and at the other site the order was A. fatua>wheat>barley. In order to explain and predict differences between years and sites, more studies are required on morphological development in relation to abiotic variables.

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IntroductionIt has recently been emphasized (14,15, 19, 20,32) that the two main environmental conditions that influence the development and health of ordinary leafy plants, as far as water relations are concerned, are (a) the capacity of the soil to deliver water to the plant roots and (b) the capacity of the aerial surroundings to permit water loss from the foliage through transpiration. The second of these conditions may be measured and recorded in a fairly satisfactory way, by means of atmometers of various forms, and numerous studies on the relations between evaporation and plant

The water-supplying power of the soil has been investigated by a num-,ber of workers during the past 30 years. The soil-point method which is used in this study was described by LIVINGSTON and KOKETSU (3). This method has since been used to study the seasonal variations in the watersupplying power of the soil with reference to rainfall, evaporation rate and the growth condition of lawn grasses by LIVINGSTON and OHGA (4) Only four of the seven plots were used in the study reported here. These included one check and three other plots which received 1.5, 2.0, and 3.0 times the normal, respectively. This "normal" is the average amount of rainfall for the months included in the growing season at Wooster, Ohio, as determined by the Weather Bureau station during the past 40 years. Thus the plot represented by 1.5 N received an amount of water which exceeded that which would normally fall on the plots by 50 per cent., the 2.0 N an excess of 100 per cent., and the 3.0 N exceeded the check by 200 per cent. Since the rainfall was very deficient during the summer of 1930 these relative values did not obtain, as will be shown later.

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