1904] Relationship between Weather and Plant Growth. 51 



to their varied sensitiveness to heat, as well as to their more or 

 less favorable situation for receiving the heat. The sensitiveness 

 ot our wild plants to heat has been determined in very few cases. 

 In our hemlock, the Iowe>«t temperature at which the chlorophyll- 

 corpuscles turn green is between 44. 6° and 53 6° F. The decom- 

 position of carbon dioxide with evolution of oxygen begins in 

 potomogeton between 50^ and 59*^, and in the eel-grass above 

 42.8"^. Even the most sensitive, i.e., the most prompt to react 

 in this way, as the larch and meadow-grass, require a temperature 

 above the freezing point, between 32.9*^ and 36.5*^ in the case of 

 the larch, and between 34.7*^ and 38.3° for the meadow-grasses. 

 From our observations we might reasonably infer that silver 

 maples, hepaticas, spring beauties and the trailing arbutus, all ot 

 which have been found blooming in March in one or both of the 

 last two springs with a maximum temperature of about 50^, rank 

 with these in readiness to react to heat stimuli; while golden-rod, 

 asters and indeed most of the Composite family must require a 

 great deal of heat. On the 6th of April, ic)03, after several cool 

 days and frosty nights, I have this record : ".Still cool and cloudy; 

 no growth." In going about the streets and the woods I had 

 noticed little or no change in the vegetation those days. The 

 aspen and alder catkins, which I noticed well shaken out on the 

 fifth, did not begin to shed pollen until the ninth, which was a 

 fine mild day, the temperature reaching 56*^. The habit of the 

 plant as regards the order of growth of stock leaves and flowers 

 is another factor conditioning the time of blooming. Plants such 

 as the hepatica, trailing arbutus, early coral-root, silver maple, 

 alder, aspen, hazel, and glaucous willow, which in the spring 

 devote their new-found energies of growth to the unfolding of 

 flowers-buds already formed the previous year, will naturally 

 bloom early; while those which, like the golden-rod and oak and 

 ash, attend first to the growth of stock and stem and leaves will 

 naturally bloom relatively late. 



A word might be said here regarding the time when the 

 various orders and families of plants come into bloom. During 

 April in these two exceptionally early springs I found 75 species 

 of plant in bloom, including two grasses and six sedges. Leaving 

 out of consideration the sedges and grasses, which I observed 



