RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 93 



1 8. Altitude affects plants through low humidities and shortness 

 of season. 



/iQ.yExtremes of any factor which are not the optimum for the 

 species tend to dwarf plants growing in them. 



20. Species are plastic in different degrees. The greatest sta- 

 bility is shown by the composites, although stable species occur 

 among other families. 



21. No laws can as yet be laid down as to the exact amount of 

 change taking place in the histology of the leaf in response to a 

 definite difference in the physical factors. Among plastic species it 

 is proportional within certain limits, but beyond these epidermal and 

 morphological modifications must be taken into account. 



VIII. BIBLIOGRAPHY 



ARESCHOUG, F. W. 



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94. Les plantes arctiques comparees aux memes Especes des Alpes et des 

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95. Recherches experimentales sur 1'adaptation des Plantes aux Climat 

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