DEVELOPMENT AND REPRODUCTION 339 



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 life and 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 

 many 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 devel- 

 opment 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 conditions; 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 

 the fixed limits for that form. When two environmental complexes are different to a 

 considerable degree but both are suitable for the development of a given species, 

 development with one complex may be very different from that with the other. 

 Thus, Bonnier's experiments showed that the same plant (Jerusalem artichoke, 

 for example) developed very differently in lowlands and in alpine regions. Also, 

 several lots of seed of the same individual plant, each lot grown under a different set of 

 climatic conditions, may produce plants that are very different, those of each lot being 

 characteristic for their own climatic complex and also different from the plants secured 

 when all lots of seed are germinated together and the seedings are reared side by side. 

 This point is of agricultural importance. 



Potato tubers are branch stems that develop underground. In darkness, tubers 

 may be caused to develop above the soil surface. Vochting was able to arrange condi- 

 tions so that tubers were produced above the soil and in light, even at the tip of the 

 shoot, in which case the carbohydrates formed in the leaves must have moved upward 



