[Chap. XL VARIATIONS AND DI\'ERSITY OF ORGANISMS 499 



One often hears that plants are well adapted to the environments in 

 which the)' grow. Any serious ecological study of a plant will most 

 probably show that it may be found in some of the habitats in which it 

 grows well — is well adapted — that it also occurs in other habitats in 

 which it barely survi\'es, but that it does not occur in habitats in which 

 it cannot survive. All this merely means that some plants grow better 

 in some environments than others do because they are different kinds 

 of plants. It will become clearer later on that a distinction should be 

 made between the popular idea that an individual plant becomes 

 adapted, and the botanist's idea that a species of plant becomes adapted. 



In one sense, however, it is true that an individual plant becomes 

 adapted, or better adapted, to a certain environmental condition. This 

 fact may be easily illustrated with seedlings, say of cabbage. If seeds of 

 cabbage are planted in three different plots, one of which is kept warm 

 and moist, another wami and comparatively dry, and the third cool and 

 either moist or drv, the seedlings will grow in all three plots. A few weeks 

 later, if the seedlings are exposed to a freezing temperature for a few 

 hours, those in the warm plot will be killed, while those in the dry and 

 cool plots may survive. They have become physiologically different. 

 The gardener says they have become "hardened." This physiological 

 conditioning of a plant, however, is not what is usuallv meant by the 

 terms acclimated and adapted. It is merely a fluctuation that is not trans- 

 mitted to the progeny. 



One usually refers to the acclimation and adaptation of a mixed popu- 

 lation of plants rather than to a single kind of plant. A few examples will 

 be discussed briefly because of the principles they exemplify. A popula- 

 tion composed entirely of the same kind of homozygous close pollinated 

 plants 'SA'ould behave like a single indixidual, provided, of course, that 

 no mutations occurred in any of the indi\'iduals. If there were mutations 

 in some of the individuals then we would no longer have a homogeneous 

 population of plants, but a mixed one of different kinds of plants. In a 

 population composed of different kinds of plants, some plants behave 

 differently than others when the environment changes. 



Examples of mixed populations of plants are: a bluegrass lawn in 

 which there are also other kinds of grasses, clover, and weeds; a forest; 

 or a field of crop plants composed of closely related species and varieties 

 of plants. 



Several kinds of changes may occur in a mixed population, but it is a 

 question whether any of them is correctly referred to as becoming ac- 



