274 PLANT-LIFE 



remarkable feats of constructive chemistry such feats, 

 indeed, as stagger the ingenuity of man. 



The growth of plants most certainly depends upon 

 nutrition and the physiological activities involved 

 therein. These may be regarded as the internal con- 

 ditions of growth, but there are also certain external 

 conditions affecting this life-process which need to be 

 taken into account. Paramount among the external 

 circumstances which stimulate and regulate internal 

 activities are temperature, light, and gravitation. 



Temperature has a most important bearing on growth. 

 Plants cannot thrive at a temperature above 50 C. or 

 below C. The degree of warmth which promotes 

 growth most successfully varies with different plants; it 

 seems to rest between 22 and 37 C. Plants accus- 

 tomed to life in high latitudes, in the Alps and polar 

 regions, may exhibit vigorous growth at a temperature 

 little above zero. It has been shown that the young 

 roots of Wheat and Barley attain their maximum of 

 growth at 22-8 C. A fall below this optimum tempera- 

 ture checks growth, and if the lower degree is main- 

 tained it will cause death. Death also ensues if the 

 optimum of heat is exceeded to any considerable extent. 

 The presence of moisture in plants has a bearing upon 

 their death by exposure to extremes of temperature. 

 Dry seeds can endure considerable extremes, such, 

 indeed, as would speedily kill fleshy ones, or normally 

 dry ones softened with moisture. The seasonal changes 

 of temperature materially affects the growth of our 

 perennial plants. In winter growth is suspended, in 

 spring it is exceedingly vigorous, in summer it begins 



