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 
may 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. 
teaumur 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 
‘alled a thermal constant in phenology. For the three growing 
