INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



225 



ture alone, other conditions being equal, are sufficient to produce differences in 

 the external appearance of certain plants. Thus, for example, the stem of 

 Mimulus tilingii grows vertically upward at ordinary temperatures, while it 

 bends or even assumes a horizontal position at lower temperatures. 



It is well known that the climate of high mountains is characterized by great 

 fluctuations in temperature, and the question arises whether this environmental 

 feature does not also play a part in producing the peculiar aspect of alpine plants. 

 To answer this question by experiment, plants from low altitudes were grown 

 from the seed in vessels that were surrounded with ice at night and were 

 exposed to the usual lowland conditions during the day, thus simulating the 

 daily temperature fluctuation observed on high mountains. Plants thus 

 grown possessed the special peculiarities of the forms occurring in alpine floras 

 (limited enlargement, short internodes, small, tough leaves, and early flowering 

 periods). 



A striking example of the influence of 

 temperature upon plant configuration is 

 found in the case of a species of acetic 

 acid bacterium {Bacterium pasteurianum) . 



Fig. 103. — Bacterium 'pasteurianum, 

 grown at 34°C. 



Fig. 104. — Bacterium pasteurianum, 

 grown at 40°C. 



Cultivated at medium temperatures this organism assumes the form of short 

 rods, usually joined together in rows or chains (Fig. 103). If a part of such 

 a culture is transferred to fresh nutrient solution and subjected to a tem- 

 perature of 4o°C., the cells elongate, after a few hours, into slender filaments 

 (Fig. 104). These filaments are sometimes as much as 150 times as long as are 

 the original rod-shaped forms. When such a filamentous culture is returned to 

 a temperature of 34°, the rod form is once more produced; the filaments first 

 develop local swellings and then the portions between these thickened regions 

 divide into the short cells of the other form. The thickened portions remain 

 unchanged, and finally die. 



The dependence of development upon temperature can be established by 

 phenological observations. To find out the temperature requirements of any 

 annual plant, the average or maximum temperature, above zero, is recorded for 

 every day from the time of planting until the complete ripening of the fruit. 

 The sum of these daily temperatures is taken to represent the amount of heat 

 necessary for the complete development of the plants in question. 



