‘ 
NEw YorK AGRICULTURAL EXPERIMENT STATION. 445 
Any knowledge, then, that will enable us to forecast the advent of 
the blooming season even though it be vague, must be helpful. 
Blooming-time is not a fixed event in the succession of life events 
of plants. If, for example, we consider leafing and blooming we 
shall find that in the northern climates the tree fruits come into 
leaf and bloom almost simultaneously and at the first wave of 
summer weather. In more southern climates many of the varieties 
of the same species will bloom first and then come into foliage. 
Similarly, the relations as to time of taking place of all life-epochs 
are somewhat dependent upon climate and without doubt other 
conditions. Of the several attributes of climate, temperature seems 
to have by tar the greatest influence in hastening or retarding the 
advent of blooming time. 
Since temperature is so easily measured, and since plants may 
be observed under the different degrees of temperature, it would 
seem that there might be some thermal constant whereby the 
advance of vegetation can be gauged in any season. The problem 
is one that has engaged the attention of students of climate and 
plant life for nearly two centuries and is worth brief mention here. 
Of the score or more of hypotheses that have been formed for 
paralleling the development of vegetation with thermometric 
values, the principal one is that of Hoffman. 
Hoffman’s! conclusion, after many years’ study of the quantity 
of heat needed for a definite phase of vegetation was: That be- 
ginning with midwinter when vegetation is dormant, the zero 
point of vegetation in his hypothesis, and for which he takes the 
first of January as an arbitrary date, if one takes the sum of the 
daily, maximum, positive temperatures of a thermometer fully 
exposed to the sun, up to a day of the attainment of any definite 
phase of vegetation, as blooming, leafing, or ripe fruit, one has a 
thermal constant which will coincide from year to year to a satis- 
factory degree. 
Relations that would follow from the above hypothesis are: 
Though a life epoch may begin at varying dates from year to year 
depending on the climate of the year, yet to reach a_partic- 
ular epoch, a plant requires an amount of heat that is constant 
from year to year. Plants may therefore be considered a means of 
measuring heat. The beginning of any life-epoch marks a certain 
"Hoffman. Comparative phenological chart for Central Europe. Peter- 
mann’s Geog. Mitth., 1881. 
