THE FORMAL CONDITIONS FOR GROWTH 69 



the percentage of water falls below the minimal limit. Similarly, active 

 vegetation begins in spring only when a sufficient temperature is reached, 

 even although all the other conditions were previously amply satisfied. 



The result produced depends upon the receptivity and power of 

 response of the particular plant at the given time, that is, upon its general 

 tone. The latter is influenced by the supply of water and of food, as 

 well as by the most varied external agencies, so that the effect of 

 temperature is not the same, at least quantitatively, upon drooping or 

 starved plants as upon turgid and well-nourished ones. Indeed, in some 

 cases changes of tone may lead to the reversal of the response to a 

 particular stimulus. Hence, the relationship between a varying factor and 

 the physiological response it produces can only be empirically determined 

 for a particular set of external conditions, and it may vary when these 

 alter. The joint effect of two factors cannot therefore always be predicted 

 from the results they produce when acting singly. 



A sufficient alteration in any one of the essential factors, co-operating 

 in the production of growth, must necessarily produce a change and 

 ultimate cessation of the latter. Thus, a rise of temperature above the 

 optimum produces a retardation and ultimate cessation of growth, 

 although the activity of respiration is much increased, while metabolism 

 may still continue after the absence of oxygen has caused the growth 

 of an aerobe to cease. Again, a plant may be rendered immotile or 

 incapable of CO 2 -assimilation by moderate anaesthetization, although re- 

 spiration persists. A complete temporary cessation of all vital functions 

 can probably only be produced by the action of extreme cold and by 

 desiccation upon resistant seeds, spores, and adult plants. According to 

 the agency producing partial or complete arrest of the vital functions 

 we may speak of heat-rigor^ cold-rigor, dry-rigor , or rigor mortis^ and to 

 indicate the agencies necessary for the production and maintenance of the 

 power of response, we may follow Sachs and use such terms as thermotonus 

 and phototonus *. 



For each essential external agency a minimum and maximum 

 limit can be recognized between which growth is possible, while the 

 optimum point is that at which it is most active. Similar curves are 

 given by the action of non-essential agencies of steadily increasing in- 

 tensity, although the cardinal points for different functions may differ 

 widely. A pronounced optimum need not necessarily be shown, however, 

 and thus the respiratory curve steadily rises with increasing temperature 

 until the plant is injured or killed, whereas the curve of CO 2 - assimilation 

 exhibits a very pronounced optimum and then falls again 2 . 



1 Sachs, Flora, 1863, p. 449. Cf. also Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 195. 



2 Cf. Errera, Essais d. phil. bot. sur 1'optimum, 1896 (Rev. de 1'Univ. de Bruxelles, T. i). 



