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 pressure, 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 conditions affect 

 the plant simultaneously, as is the case in nature." For example, the climato- 



FlG. 161. — Achyrophorus quitensis, an alpine plant. {% natural size.) 



logical 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 plants of high moun- 



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

 Sitzungsber. '(math.-naturw. Kl.) K. Akad. Wiss. Wien ioi 7 : 487-S48. 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. 256.) 



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