70 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



The curves for different functions naturally do not necessarily follow 

 precisely the same course. Thus the respiratory curve may not have 

 reached its maximum at a temperature at which growth has nearly or 

 quite ceased. The best conditions for development will, however, not 

 always be those at which growth is most active, and in fact an excessive 

 development of the vegetative organs hinders or prevents flowering. 

 Again, certain fungi and algae only complete their life-cycles under 

 the stimulus of changes in the external conditions. There is, therefore, 

 a certain biological optimum for the most advantageous development, but 

 this optimum varies according to the ends and purposes in view l . For 

 example, the optimal conditions for the production of Muter-yeast 

 differ widely from those for the formation of a sporangiferous mycelium. 

 These conclusions apply equally well whether all the factors are vari- 

 able, or all but one are constant, for in the latter case the variable 

 factor will determine whether the optimal activity of growth is or is 

 not reached 2 . 



The cardinal points are liable to slight variations, and hence can only 

 approximately be determined. Growth, for example, continues for a time 

 under conditions which ultimately cause a complete cessation of all vital 

 activity. This cessation follows more or less rapidly according to whether 

 the permanent maximum is largely or only slightly surpassed. In the 

 case of a temporary anaerobe, growth may continue for a longer or shorter 

 time in the absence of oxygen, according to the character of the food- 

 material and of the organism itself. Similarly, the growth of organs 

 requiring light may not be pathologically disturbed until after several 

 days' darkness. This power of withstanding fatally injurious influences 

 for a short time is of great importance for the maintenance of plants in 

 nature. Indeed, an organ which was injured by a few hours' absence 

 of light could not survive the alternation of night and day. 



The position of the maximum and minimum points is largely dependent 

 upon the power of accommodation the plant possesses, and it is indeed 

 possible to gradually accustom plants and animals to doses of poisons which 

 previously would have been fatal. Similarly, plants may accommodate 

 themselves to concentrated media, to subminimal or supra-maximal 

 percentages of oxygen, and even to a certain extent to extremes of 

 temperature. A change in the external conditions may produce more 

 or less corresponding internal alterations, and hence may cause a slight 

 or a pronounced displacement of the cardinal points for a particular agency. 

 This is well illustrated by the fact that the resistance to heat, and to other 

 influences as well, is greatly increased by desiccation. 



1 Schimper, Pflanzengeographie, 1898, p. 50. 



3 Cf. \Vollny, Forsch. a. d. Gebiete der Agriculturphysik, 1897, Bd. xx, p. 53. 



