220 



INFLUENCE OF TEMPERATURE 



So, too, the growth of the seedlings is influenced in a similar 

 manner, the temperatures, however, being somewhat lower, especially 

 as regards the maximum temperature at which growth occurs. This 

 is usually about 38, but with maize, pumpkins and tropical plants, 

 of course, may be higher up to about 45 (113 F.). Certain low 

 forms of plant-life have limits much below and much above these 

 temperatures. Some algce will grow in sea water at C. or even 

 below, while some thermophilic bacteria flourish at 75 C. and have 

 -an optimum temperature of growth about 70 C. 



The rate of growth of seedlings is found to be very small near their 

 minimum temperature and to increase gradually with a rise of tem- 

 perature, the rate of increase rising rapidly as the optimum temperature 

 is approached and diminishing very rapidly soon after this is passed, 

 until it finally ceases at the maximum temperature. 



In South Africa, temperatures are liable to great and rapid oscilla- 

 tions. The shade temperatures in the afternoon of a clear sunny 

 winter's afternoon may reach 80 F. (26'6 C.), while shortly before 

 sunrise there may be a keen frost. Now plants are able to adapt 

 themselves to extremes of temperature, provided the changes occur 

 gradually, but such rapid alternations must be very injurious to almost 

 all plants. In the summer, both day and night, temperatures are much 

 higher, and the temperature of the surface soil and of the leaves of 

 plants must often be high, probably far above the maximum temperature 

 of growth. 



Thus, in the case of winter crops (grown with irrigation), it must 

 often happen that the time during which the plant is at or near its 

 optimum temperature must be restricted to comparatively short periods 

 in the mornings and evenings, during the rest of the twenty-four 

 hours the temperatures being too low or too high to allow of much 

 growth. 



Moreover, the adjustment of the organs of the plant to suit these 

 rapid changes of temperature must be difficult. 



We consequently find that many English plants, which in the 

 English climate are able to withstand greater extremes of temperature 

 (but more gradual in their alternation), will not live through a Trans- 

 vaal winter, even when well watered. This, for example, is the case 

 with many grasses. 



Any circumstance which tends to increase the suddenness of the 

 transition from the too low to the too high temperatures will still further 

 retard the growth of plants. Such a circumstance is the screening off 



