Some factors in plant competition

SUMMARY. 1 The mutual action of one plant on another when grown in juxtaposition, usually known as competition, is a very complex phenomenon. Among the factors which come into play are competition for food, water and light, and also the possible harmful effect due to toxic excretions from the roots. 2 When the food supply is limited the dominant factor of competition is that of food and in particular the amount of available nitrogen. Other things being equal the total growth as measured by the dry matter produced is determined by the nitrogen supply, irrespective of the number of plants drawing on the resources. 3 With limited food supply the efficiency index of dry weight production decreases with the number of plants, as the working capacity of the plant is limited by the quantity of material available for building up the tissues. 4 The decrease in light caused by overcrowding is a most potent factor in competition even when an abundance of food and water is presented to each individual plant. With barley the effect of light competition is (a).? To reduce the number of ears. (b).? To cause great irregularity in the number of tillers produced. (c).? To reduce the amount of dry matter formed. (d).? To encourage shoot growth at the expense of root growth, thus raising the ratio of shoot to root. (e).? To increase the, variation in the efficiency indices of dry weight production of a number of crowded plants, lowering them on the average. (f).? To decrease the power of the, plants to make use of the food supplied to the roots, as evidenced by the, total quantity of nitrogen taken up by similar numbers of plants when spaced out and crowded. 5 With adequate illumination (in barley) there is a tendency towards the production of a standard type of plant in which the relation between the number of tillers and ears, dry weights, efficiency indices, and ratios of root to shoot approximates within variable degrees to a constant standard. With overcrowding this approximation entirely disappears.