CHAPTER X 

 PHYSICAL FACTORS: LIGHT 



Any part of the environment of a plant that affects in any way the 

 life of the plant may be spoken of as an environmental factor or 

 ecological factor. The environment of any plant is partly living and 

 partly non-living. The living part of the environment consists of 

 plants and animals. These may be spoken of collectively as biotic 

 factors. They have been discussed at some length in the chapters on 

 Symbiosis and will be dealt with further in some of the succeeding 

 chapters. 



The non-living part of the environment consists of a variety of 

 influences that may be spoken of collectively as physical factors. 

 The more important of these will be discussed in the present and the 

 immediately following chapters. 



71. Gravity.— An environmental factor becomes important eco- 

 logically as it varies from place to place and in its effect upon dif- 

 ferent kinds of plants. If all factors produced the same effects on all 

 plants everywhere there would be no science of ecology, just as there 

 would be no science of human sociology if the environmental condi- 

 tions in which human beings live were universally the same. 



Gravity is the one universal factor. Its action is practically the 

 same on all parts of the earth's surface. Since it is a universal factor 

 it is not of very great importance ecologically, though it is very 

 important physiologically. Generally speaking the effect of gravity 

 is to cause roots to grow downward and stems to grow upward, but 

 it is a curious fact that the directions of growth of the lateral roots 

 and the lateral shoots are also directed largely by gravity. The 

 advantage of this phenomenon is, of course, clear enough, since it 

 would be a distinct disadvantage to a plant to have all its roots grow 

 straight downward and all its stems straight upward. As long as 

 the main shoot and the main root are intact the lateral branches seem 

 to be inhibited from growing straight upward or straight downward, 

 since if the main shoot is cut off one or more of the stronger lateral 

 branches will then assume the vertical direction. This whole 

 phenomenon is still very imperfectly understood and may not be 

 due entirely to gravity. 

 (118) 



