SEASOXA'L ACTIVITIES OF PLAXTS. 221 



in the position and relation of the critical points ensues. A cold 

 fatal to the active seedling will be found near the freezing point 

 of water, and l)ut slightly below the infra-oytimum . the optimum 

 will be found to lie between 80 degi'ees and 98 degrees F., the 

 supra-optimum and cessation of growth will be found between 

 100 degrees F. and 120 degrees F. for most plants, although 

 many species, especially those native to the desert, range higher, 

 while a fatal heat comes within a few degrees above the supra- 

 optimum. 



As the plant nears maturity, the tissues harden, the proto- 

 plasm becomes more highly granulose and denser, and has an 

 altered chemical composition, by which it again becomes less sus- 

 ceptible to alterations, and again the cardinal points take posi- 

 tions more widely separated from each other, and in the seed 

 are again able to endure any cold which they may encounter. 



THERMAL REQUIREMENTS OF A PLANT. 



This brings us at once to the consideration of the practi- 

 cability of some estimation of the thermal constant of any form, 

 or the amount of heat necessary for its seasonal or cyclical de- 

 velopment. The first effort toward fixing any such standard 

 appears to have been made by Keaumur, the inventor uf the ther- 

 mometric scale which bears his name. He adopted the sum of the 

 mean daily temperatures, as recorded by his thermometer in the 

 shade, as an index of the amount of heat required to bring a 

 plant to any given stage of development, using averages of the 

 daily maximum and minimum to obtain his mean daily temper- 

 atures. According to Abbe, Reaumur calculated that the sum 

 of the averages constituting the heat exposure of a plant at his 

 locality in France during the 91 days of April, May and June, 

 1734, to be equivalent to 1160 degrees C, but in the following 

 year it amounted to only 1015 degrees C. 



Adansonn disregarded all temperatures below freezing, tak- 

 ing only the sum of the positive temperatures on the centigrade 

 scale, and beg-an the summation of heat exposures thus derived 

 with the 1st day of the calendar year for any given season. 



