237 



plant were taken, and it was found that the ratios are very nearly the 

 same for all jDlants; therefore the ratio given by the best series, viz, 

 for Lonicera alplgena 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 w^ould haA^e 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 shoAvn that such agreements do not always recur. 



In the Zeitschrift for 1881 Hofl'mann 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 

 Avhose lowest temperature was — 31° Reaum. than did the plants in 

 the lowlands; the shady side of the tree suffered less than the sunny 

 side. It i& indifferent Avhether the sudden rise in temperature is 

 caused by great solar rays or by a sudden warm wind ; the sudden rise 

 from — 12° Reaum. to -|-13° Reaum. is as bad for plants as the sud- 

 den rise from — 20° Reaum. to -(-5° Reaum.; 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 givew 

 the results of observations at Giessen for 1880. He finds that the 

 blossoming in spring-time is so subject to disturbances by frost that the 

 midsummer and autunmal 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 injur}'^ 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. 



