237 
plant were taken, and it was found that the ratios are very nearly the 
same for all plants; therefore the ratio given by the best series, viz, 
for Lonicera alpigena was taken as a standard and applied to the 
series for the other plants, so as to reduce all observations with the 
later thermometers back to agreement with what would have been 
given by the first thermometer had it not been broken. The ratios 
of the sums observed at Giessen with the new thermometer as com- 
pared with the sums observed at Frankfort, also with a similar new 
thermometer, agreed closely for all the plants, and as the two new 
thermometers agree closely with each other when placed side by 
side, it was assumed that the ratios thus obtained represent the reduc- 
tion from the climate of Frankfort to that of Giessen. Adopting the 
same standard plant and the ratio of its sums for any place to its 
sums at Giessen as the standard ratio, all the sums for plants at that 
place can be reduced to what would have been given by the same plants 
at Giessen and to what would have been given by the first Giessen 
thermometer. Although these reductions are very arbitrary, yet the 
agreement of the sums thus computed for Giessen with those actually 
observed was quite close. But, as we shall see, subsequent years of 
observations have shown that such agreements do not always recur. 
In the Zeitschrift for 1881 Hoffmann shows that it is not the low 
temperatures but the subsequent too rapid thawing that injures most 
plants; thus the hill stations suffered less at the close of a period 
whose lowest temperature was —31° Réaum. than did the plants in 
the lowlands; the shady side of the tree suffered less than the sunny 
side. It is indifferent whether the sudden rise in temperature is 
caused by great solar rays or by a sudden warm wind; the sudden rise 
from —12° Réaum. to +13° Réaum. is as bad for plants as the sud- 
den rise from —20° Réaum. to -+-5° Réaum.; the amount of injury is 
proportional to the extent and to the suddenness of the rise. 
In the same volume of the Zeitschrift (p. 330) Hoffmann gives 
the results of observations at Giessen for 1880. He finds that the 
blossoming in springtime is so subject to disturbances by frost that the 
midsummer and autumnal phases of vegetation are more proper to 
show the accuracy of his methods. He finds that these later phases, 
as observed at Giessen (1866-1869), when reduced to the new stand- 
ard thermometer at Giessen agree within 1 per cent with the actual 
observations of 1880 at that place. For plants that bloom in the 
spring he finds that if these are protected from injury by frost by 
placing them under glass covers there is then a better but still unsat- 
isfactory agreement between the observations at Giessen and Frank- 
fort. On computing the mean temperature of the air in the shade for 
the dates of blooming at Giessen he finds no apparent connection, so 
that from the date of blooming we can not infer the mean tempera- 
ture of that day nor can we reason from the temperature to the date. 
