4.5O CHARACTER OF VEGETABLE VITALITY. CHAP. XI. 



and the Oak and Ash, which are always the latest 

 among trees, in the beginning or towards the middle 

 of May. Many annuals do not come up till after 

 the summer solstice ; and many Mosses not till 

 after commencement of winter. This gradual and 

 successive unfolding of the leaves of different plants 

 seems to arise from the peculiar susceptibility of the 

 species to the action of heat, as requiring a greater 

 or less degree of it to give the proper stimulus to the 

 vital principle. But a great many circumstances 

 will always concur to render the time of the unfold- 

 ing of the leaves somewhat irregular ; because the 

 mildness of the season is by no means uniform at 

 the same period of advancement ; and because the 

 leafing of the plant depends upon the peculiar de- 

 gree of temperature, and not upon the return of a 

 particular day of the year. Hence it has been 

 thought that no rule could be so good for directing 

 the husbandman in the sowing of his several sorts 

 of grain as the leafing of such species of trees as 

 might be found by observation to correspond best 

 to each sort of grain respectively, in the degree of 

 temperature required. 



A signal to Linnaeus, who instituted some observations on 

 bandmaii. the subject about the year 1750, with a view chiefly 

 to ascertain the time proper for the sowing of Barley 

 in Sweden, regarded the leafing of the Birch-tree as 

 being the best indication for that grain, and recom- 

 mended the institution of similar observations with 

 regard to other sorts of grain, upon the ground of 



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