THE PLANT AND ITS ENVIRONMENT 177 



of advantage to both though they are not intiniati! enough to 

 class as true syni])ioses. Most notable are those between flower- 

 ing plants and insects, whereby the plant derives the benefit 

 of cross-pollination and the insect a supply of nectar; and the 

 numerous cases where fruits are attractive to animals by furnish- 

 ing them with food, and are in turn distributed widely by these 

 animals, with consequent benefit to the plant species. These 

 two types of relationship are very numerous and fascinating and 

 have long been the subject of investigation by botanists. We 

 shall describe them more fully in a later chapter. 



In this brief discussion of the influence upon plants of the 

 various factors in their environment, we have merely outlined 

 some of the important aspects of the problem which underlies 

 the whole science of ecology. No one of these factors is para- 

 mount, and the reaction of the plant is to the whole series of them 

 (Fig. 95). The direction of growth, for example, may be influ- 

 enced both by gravity and by light, and the result is determined 

 by the relative strength of these two stimuli under the particular 

 conditions existing in a given case. In the same way, the 

 distribution of a given plant species over the earth's surface is 

 determined not alone by soil conditions or moisture or tem- 

 perature or parasites, but by the whole series of environmental 

 factors taken together. The essential point to remember is 

 that the structure and activities of any plant are not entirely 

 controlled either by the environment in which it lives or by the 

 specific and inborn characteristics of its protoplasm, but are 

 results of an interaction between these two forces. Plants 

 belonging to different species will develop and function very 

 differently even under environments which are identical; and 

 plants entirely similar will become very different if grown under 

 different environments. We are here facing the ancient problem 

 of Heredity versus Environment, which confronts man in so many 

 fields of incjuiry. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



487. Give an example of the response of a plant to its environment 

 which is regulatory in character. 



488. What do we mean by saying the "no nervous system has been 

 differentiated" in plants? 



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