254 PLANT PHYSIOLOGY AND ECOLOGY 



a consequence, the unsuccessful competitor disappears entirely, 

 or is so handicapped that it produces fewer or less vigorous seeds. 



Competition for both water and light is the rule when plants 

 of varying height are intermingled. This is always true when the 

 plants are broad-leaved or branched. In the case of grasses, 

 many mats, rosettes, etc., competition for light is relatively unim- 

 portant on account of the size or position of the leaves. With 

 such forms as the leafless sedges and rushes, it seems to be entirely 

 lacking. Plants which grow in saturated soils or in water appar- 

 ently do not compete for the latter, though it is probable that 

 a new factor, air content, enters the question. Finally, it is 

 possible for plants to be densely crowded, and still not compete 

 with each other This is nicely illustrated by duckweeds which 

 often completely cover the surface of ponds and streams. The 

 tiny fronds are on an equality with respect to light, and the water 

 supply is far in excess of the demand. 



268. Competition between parents and offspring. The simplest 

 kind of ordinary competition is that in which the individuals 

 belong to the same species. The various individuals of a family 

 show relatively slight differences in height, Avidth, leaf expanse, 

 and root surface. Some have surfaces which are larger or better 

 situated for recei\'ing water or light, and the others are thus placed 

 at a disadvantage. The former receive more than their share 

 of water or light, or of both. The reaction which they produce 

 upon these factors affects the plants subject to it. The usual 

 result of such competition is great variation in the height, branch- 

 ing, and leaf area of the different individuals, and the inability 

 of many to produce flowers. This is particularly true of annuals 

 and of perennials belonging to the same generation. In the 

 competition between the parents and offspring of the same per- 

 ennial species, the former usually have a decided advantage. 

 The younger plants are often unable to thrive, or even to germinate, 

 and they finally disappear, leaving a free space beneath and about 

 the stronger parents. Similar individuals make practically the 

 same demands upon the habitat, and adjust themselves least 

 readily to their mutual reactions. The more unlike plants are 

 the greater the difference in their needs. Hence some are able to 

 adjust themselves to the reaction of others wdth little or no dis- 

 advantage. From this is obtained the primary law of competition, 

 i.e., competition is closest when the individuals are most similar. 



