34 



THE AMERICAN BOTANIST. 



that the young and tender vegetation of spring is easily 

 destroyed by frost, and the important role plaj^ed by the 

 physical properties of water in the process are now well 

 understood. Freezing depends on whether the affinity of 

 the protoplasm for water is less than the molecular forces 

 causing crystalhzation. If the affinity of the protoplasm 

 for moisture is overcome, the water exudes from the cells 

 into the intercellular spaces, w^here it crystallizes, and 

 death results from the loss of water at a time when meta- 

 bolic processes are at a minimum. From the point of 

 agricultural interests the first and last killing frosts are 

 events of much practical importance, which often exert a 

 marked influence on the yield of crops. They determine, 

 in fact, the limits of the growing periods of most crops, 

 and the immense damage often done by late frosts has led 

 to strenuous efforts to secure means of protection. Conse- 

 quently the last killing frost, as a natural event easily de- 

 termined, with a distinctive influence on vegetation, ma}^ 

 correctly be considered as marking the final passage of 

 winter, the point of beginning of uninterrupted progress 

 in the growth of vegetation — the advent of spring. 



Many facts of great interest appear in this connection. 

 The earliest date recorded is January 20, at San Diego; 

 since, however, the people of south California claim per- 

 petual summer for that delightful climate, we can hardly 

 permit them to enjoy the pleasures of spring. By Febru- 

 ary 1 spring has commenced along the entire coast line of 

 the Gulf of Mexico and over Florida, but its progress dur- 

 ing February and March is rather slow. The line for 

 April 1 starts from a point on the Atlantic coast some dis- 

 tance north of Norfolk, Va., passes southwest through 

 eastern North Carolina, curves around the south end of 

 the Alleghenies, passes through Nashville, Tenn., above 

 Cairo, 111., and thence takes a wide southward bend below 

 the Rocky mountain region ; in the extreme west it passes 

 directly northward along the v^sestern edge of the Sierra 

 Nevada Mountains, and leaves the United States north of 

 Seattle, Wash. Thus spring prevails over the entire Pacific 



