168 



responding with the formation of the leaf is its ripening and fall 

 from the tree, which Fritsch adds to his list of epochs, viz : 



(7) The fall of the leaf or the time when the tree has shed fully 

 one-half of its leaves; as the wind and heavy rains accelerate this 

 process the date is liable to considerable uncertainty independent 

 of the vitality of the plant. Therefore, in this, as in all other epochs, 

 Fritsch, in endeavoring to lay the foundations of the study, rejected 

 those cases in which any unusual phenomenon, such as wind or 

 drought or insects, had a decided influence on the observed dates. 



Many plants blossom a second time in the autumn, although they 

 ma}^ not ripen their fruits; therefore in special cases Fritsch adds an 

 eighth epoch, viz : 



(8) The second date of flowering. Of course it is understood that 

 if the second flowering is brought about artificially, as by irrigation, 

 pruning, or mowing, that fact must be mentioned. 



"When the flowers blossom in clusters, such that the individuals 

 are lost sight of in the general effect, then, in addition to the first 

 flower, we note the following item : 



(9) The general flowering or the time when the flowers are most 

 uniformly distributed over the plant. 



For 118 varieties Fritsch gives in detail the phenomena that char- 

 acterize the date of the ripening of the fruit. He also gives an equally 

 elaborate system of observations on birds, mammals, fishes, reptiles, 

 and insects, and especially the mollusks or garden snails and slugs. 



THE RELATION OF TEMPERATURE AND SUNSHINE TO THE 

 DEVELOPMENT OF PLANTS— THERMOMETRIC AND ACTINO- 

 METRIC CONSTANTS. 



Reaumur was the first to make an exact comparison of the different 

 quantities of heat required to bring a plant up to the given stage of 

 maturity, and since then many authors have written on this subject. 



I will here give a brief summary of views that have been held by 

 prominent authorities as to the proper method of ascertaining and 

 stating the relation between temperature and the development of 

 plants. 



Reaumur (1735) adopted simply the sum of the mean daily tem- 

 peratures of the air as recorded by a thermometer in the shade and 

 counting from any given phenological epoch to any other epoch. 

 He employed the average of the daily maximum and minimum as a 

 sufficiently close approximation to the average daily temperatures, 

 and evidently in the absence of hourly observations any of the recog- 

 nized combinations of observations may be used for this purpose. 

 Reaumur found from his observations that the sum of these daily 

 temperatures was approximately constant for the period of develop- 

 ment of any plant from year to year; hence this constant sum is 

 called a thermal constant in phenology. For the three growing 



