SEASONAL ACTIVITIES OF PLAINTS. 227 



'New York Botanical Garden and the meadow of the herbaceous 

 grounds, not more than 500 yards distant made in 1900 and 

 1901. The data obtained shows that the meadow carpet re- 

 ■ceived 78836 hour-degrees of heat during the year ending April 

 1, 1901, extending over 7282 hours of exposure to temperatures 

 above the freezing point, while the hemlocks received b;it 68596 

 similar units with an exposure to temperatures above the freez- 

 ing point for 7024 hours. The meadow was exposed to 5569 

 units of temperature below the freezing point and the hemlocks 

 5791 units. The herbaceous grounds were below the freezing 

 point for 11:78 hours, and the hemlocks for 1736 hours. 



Here, then, in two localities within rifle shot are to be found 

 two habitats for plants in which the difference in the length of 

 the season as indicated by the number of hours above the freez- 

 ing point amounts to nearly eleven days, the total number of 

 lieat units in the meadow being 13 per cent, in excess of that of 

 the forest. The maximum and average maximum of the meadow 

 are higher, and the minima and average minima are lower, the 

 mean average of the hemlocks being lower, however, than that of 

 the meadow. The value of such observations is greatly en- 

 hanced by the fact that they represent a comparatively narrow 

 Tange of variation. Thus, in the several years in which observa- 

 tions were made as to the matter, the length of the period be- 

 tween the last frost of spring and the first of autumn lay between 

 167 and 171 days in the ISTew York Botanical Garden. 



The data upon which these conclusions rest are shown in the 

 accompanying tables. The amounts given under "total expos- 

 ure" represent the product of the number of degrees above freez- 

 ing point multiplied by the number of hours. The value of the 

 centigrade-hour-unit represented in such amounts is 9/1600, 

 and of Fahrenheit hour units 81/8000. The number of hour 

 units is therefore to be found by simple division. -|- indicates 

 temperature exposures above freezing. — below freezing. 



