VIII. 1. THE NECKLACE POPLAR. 249 



age year, the aspen began to flower, at Middletown, April 1st, 

 the large poplar, about the 4th, and three others, on the 9th. 



Dr. Barratt has observations on the period of flowering through 

 fifteen degrees of north latitude, which give three months as the 

 difference in the time of beginning, or one month's difference 

 for five degrees of latitude, which is equal to six days for one 

 degree ; so that spring goes northward at the rate of one degree 

 in six days, or ten miles a day. This is the average for fifteen 

 degrees, and would give a difference of five days between Mid- 

 dletown and Boston, the difference of latitude being 48' 12". 

 The actual difference is greater, being from six to ten days, 

 showing that the advance of spring is not uniform throughout 

 every part of the fifteen degrees. The difference against Boston 

 is probably owing to the influence of the chilling north-east 

 winds which prevail at that season of the year. 



Dr. Barratt's conclusion is not far different from that reached 

 by Dr. Bigelow,* from a comparison of the times of flowering of 

 several common plants, in various parts of the United States 

 and Canada, in the spring of 1817. Dr. Bigelow made a differ- 

 ence of two months and a half for a difference of latitude of 13° 

 45 / , which would be three months for 16° 30 / . Generalizations of 

 this kind, to be valuable, must be cautiously made, drawn from 

 the average of a large number of species, and a somewhat long 

 series of years. In the data furnished by Dr. Bigelow's corres- 

 pondents, if an inference were drawn from the apple and pear 

 alone, the difference in the season between Charleston and Mon- 

 treal, whose difference of latitude is 12° 51', would be only one 

 month and twenty-one days ; if from the flowering of the blood 

 root, it would be only one month and eleven days, conclusions 

 widely different from those drawn from the average of all the 

 species observed. 



Sp. 5. The Necklace Poplar. P. monilifera. Aiton. 



Leaves figured in Michaux, Plate 96. A leafy branch is figured by Abbott, 

 Insects, II, Plate 71, with the Kitten Moth. 



This tree has an erect or slightly bending trunk, tapering 

 gradually to a height of fifty or sixty feet, and covered with a 



* See Memoirs of the American Academy, Vol. IV, p. 77. /\v ---'•/ 



33 M"" 9 %< 



