170 Some Factors in Plant Cotnjjetition 



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. 



(/) 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. 



