DEVELOPMENT AND REPRODUCTION 337 



Summary 



i. Influence of External Conditions on Development. — The development of a 

 plant is of course made up of all of its growth activities considered together. The exter- 

 nal or internal conditions that influence growth also influence development. As the 

 plant develops, its internal conditions (its physiological characteristics) are continually 

 changing, the conditions of the natural surroundings are also always in a state of 

 fluctuation, and consequently the relations between internal and external conditions are 

 likewise continually varying. It is these relations that really determine the develop- 

 mental processes. With a given kind of internal complex, a certain set of environ- 

 mental conditions would produce a certain kind of growth. If either the internal or 

 external conditions were markedly different, the kind of growth would be corres- 

 pondingly different. Thus, different species in the same environment develop differ- 

 ently, and different individuals of the same species, but in different environments, also 

 develop differently. Finally, present internal conditions, or characteristics, are the 

 results of past internal and the past environmental conditions, acting together. These 

 relations are somewhat complex, but it is clear that we may not say that any plant 

 response, or any form of development, etc., is exclusively brought about by either the 

 internal or the external conditions. Both sets of conditions are of course necessary 

 for growth, and the two sets always act simultaneously. 



Environmental complexes that are favorable to the development of one kind of 

 plant are not favorable to that of another, sufficiently different, kind. Environ- 

 mental complexes may therefore be said to be adapted to the development of those 

 plants that can thrive under their respective influences. Thus, American desert 

 conditions are very delicately and nicely adapted to the growth and reproduction of 

 certain kinds of cacti and other spiny shrubs, but they are not at all suited to the 

 development of the spiny roses found growing plentifully in the more humid regions of 

 North America. The conditions of the humid regions, on the other hand, are well 

 adapted to the development of these roses, but are not adapted to the development of 

 the desert cacti. The present conditions of desert and humid regions have been 

 brought about by a long evolutionary series of climatic and physiographic changes, 

 leading directly to the present characteristics of these regions, a series of changes that 

 began long before there were any plants. 



In a similar way, plants that thrive with one set of environmental conditions do not 

 thrive at all under another, sufficiently different, set. It follows that plant forms may 

 be said to be adapted to the particular environmental complexes under which they 

 thrive. The internal conditions characteristic of existing plant species have been 

 brought about by a long series of evolutionary steps, leading directly to the present 

 species, a series of steps that began with the inception of terrestrial life, long after the 

 corresponding climatic and physiographic series of evolutionary changes had been 

 started on its predetermined way. 



It should be added that physiographic and climatic evolution has, in some cases, 

 been greatly influenced by organisms, and that plant evolution has always been influ- 

 enced by physiographic and climatic evolution; the two lines of evolution are inter- 

 woven, and they have operated together to bring about present environments and 

 existing plants. 



While each species or form of plants requires for its development climatic and soil 

 conditions that lie within certain definite limits, each can thrive under any one of a 

 number of rather different environmental complexes, so long as all of these lie within 

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