CHAPTER VI 



DEVELOPMENT AND REPRODUCTION 



§i. Influence of External and Internal Conditions on Development. — The 



form and the arrangement of the parts of plants are dependent upon external 

 conditions to a very marked degree. According to the conditions under which 

 they develop, plants vary in their external form as well as in their internal 

 structure, and many peculiarities in configuration that appear to be 

 brought about solely by internal conditions are mainly the result of the ex- 

 ternal conditions that prevailed during the period of development. It has al- 

 ready been seen (Part II, Chapter III) that each external condition — such as 



heat, light, atmospheric pres- 

 sure, humidity, gravitation, 

 and the supply of nutrient 

 material — exerts an influence 

 upon plant growth, and 

 consequently upon both 

 external form and internal 

 structure. This influence is 

 of course more pronounced 

 when a number of different 

 environmental c o nd i tions 

 affect the plant simulta- 

 neously, as is the case in 

 nature. a For example, the climatological conditions of high mountains are very 

 different from those of lowlands, and alpine plants differ in a corresponding 

 way, in form as well as in structure, from those growing at lower levels. 1 The 

 predominating plant of high mountains have more or less reduced stems, relatively 



1 Wagner, A., Zur Kenntnis des Blattbaues der Alpenpflanzen und dessen biologischen Bedeutung. 

 Sitzungsber. (math.-naturw. Kl.) K. Akad. Wiss. Wien ioi 7 : 487-548. 1892. 



The relations of external and internal conditions to growth and development are far too 

 complex to be treated satisfactorily in the space devoted by the author to this aspect of 

 physiology. In this chapter, as well as in the preceding ones devoted to growth, the author has 

 frequently secured brevity of treatment by slighting fundamental principles and letting the 

 presentation be largely a series of superficial, or at least incompletely analyzed, examples 

 or illustrations from observations. Although it seems to the editor that this part of physiology 

 really merits just as thorough analysis and just as careful thought as does respiration, for 

 instance, still no attempt has been made to improve the analysis or to better the presentation in 

 this chapter. A somewhat more analytical point of view is taken in the editor's very brief 

 treatment presented in the summary. A more thorough attempt to bring out the fundamental 

 principles of these ecologico-physiological considerations is to be found in Part II of Livingston 

 and Shreve's book on climate and plant distribution, especially pages 97-148. (See note a, 

 p. 258.) 



324 



Fig. 161.- 



-Achyrophorus quitensis, an alpine plant, 

 natural size.) 



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