THE AMERICAN BOTANIST, 



33 



one possible for the purpose of a graphic representation. 

 The endeavor is made at the same time to select an iso- 

 therm which has some distinct relation to the phenomena 

 of plant growth. The onl3^ attempt of this kind with 

 which I am acquainted was made by Prof. Mark W. Har- 

 rington, in Harper's Magazine for is/Laj, 1894. Prof. Har- 

 rington accepted as the criterion the advance of an iso- 

 thermal line of 43.8° F., and endeavored to establish a 

 causal relation between the adopted temperature and the 

 beginning of development in plants, in the following 

 words : 



* 'Botanists state that the temperature of 43.8° F. is 

 that at which the protoplasmic contents of the vegetable 

 cell find the limit of their activit3^ When the temperature 

 falls below that point the protoplasm becomes inactive, 

 though it is not dead until the fall is from several to manj 

 degrees lower, depending on the species of plant. When 

 the temperature rises and reaches that point the proto- 

 plasm awakens, and as it passes above 42.8° F. the cells 

 begin to grow and multiply. The advent of spring may 

 be considered to take place at the advent of an isotherm 

 1° higher, or 43.8° F." 



The statement seems to me to be open to serious criti- 

 cism, and the isothermal line adopted is really quite arbi- 

 trary and with very little relation to the phenomena it is 

 supposed to mark. It can only be conceded that the rapid 

 and agreeable change from the dull gray of the forests to 

 a refreshing green does not occur much below that tem- 

 perature, but otherwise it corresponds with no particular 

 stage of vegetative growth. 



A more practical criterion of the advent of spring is 

 certainly to be found in the average date of the last killing 

 frost or occurence of a minimum temperature of 32°, an 

 event which has been determined with considerable accur- 

 acy for a large number of stations throughout the United 

 States. A killing frost is defined as one which will cause 

 the death of relativelj^ hardy vegetation, including nearlj^ 

 all the plants of ordinary cultivation. It is well known 



