118 THE STUDY OF PLANT COMMUNITIES ' Chapter VI 



These introductory statements refer to insolation as a whole. We 

 may now more conveniently consider separately the visible portion 

 of the spectrum known as light, and those longer, invisible wave 

 lengths known as heat, whose presence or absence are expressed 

 as temperature. 



FlG. 61. Circle of illumination, areas of daylight and darkness, angles of 

 sun's rays at different latitudes, and differences in areas affected and thickness 

 of atmosphere penetrated at time of summer solstice.— Adapted from Ward 

 and Powers. 265 



TEMPERATURE 



General Plant Relationships.— Each living thing is restricted to 

 a definite temperature range, which may be quite dissimilar for 

 different species and, depending largely upon the amount of water 

 in the protoplasm, may vary for individuals of a species. The wide 

 range of tolerance among species is illustrated, on the one hand, 

 by subarctic conifer forests where -80° F. has been recorded and, 

 on the other, by desert plants that withstand temperatures of 130- 

 140° F. Dormant structures such as seeds and spores are practically 

 without water and can, therefore, withstand the widest tempera- 

 ture variations and extremes. 



Plant injuries from temperature changes are most often the 

 result of freezing, which desiccates the tissues when the pure water 



