baker's north Yorkshire:. 235 



bursa-pastoris begin to germinate at 33 or 34 degrees of Fahren- 

 heit's thermometer, but the seeds of Wheat require a temperature 

 of 38, and the seeds of Flax of 40 degrees before they will begin 

 to swell. Willows and Poplars will sprout at a low temperature, 

 whilst Vines, Liriodendrons, and Magnolias need a much higher 

 one before their buds will begin to unfold. Each plant has, as 

 it were, its own special zero, all degrees of temperature below 

 which exercise, at any rate, no favourable influence upon its 

 growth. So that not only does the development of species take 

 place at different times and seasons, but we see also that the 

 temperatures of even the same season of development, for each 

 species require to be specially divided between three separate 

 divisions, those which are too low to do it any good, those which 

 are more or less useful to it, and those v.hich are too high to do 

 .t any good. And from this it results that a comparison between 

 two different localities, not only of the mean temperature of the 

 year, but even of the mean temperature of any particular month 

 or season, will often furnish results which will hold good only 

 with much exception so far as the plants which they produce, and 

 as some species more than others, are concerned. 



So numerous and complicated are the influences which inter- 

 fere to prevent the attainment of precision, that it seems to me 

 by far the safest course not to attempt to sjjcak of particular sums 

 of temperature which species need in order to develop them- 

 selves, and that if we try to do so it is more likely to mislead 

 and to confuse than to help us. If we wish to express in the 

 form of a generalisation the bearing in our climate of temperature 

 upon species-distribution, we must apparently say, that species 

 are usually limited by cold, operating c\\.\\kiv positively, that is by 

 the extreme colds or sudden falls of temperature in winter or the 

 latter part of the autumn, or by the frosts of spring killing their 

 young shoots and flower-buds, this principally applying to trees 

 and shrubs, and especially to evergreens ; or by cold operating 

 ni'i:;atively, if we may so speak, that is to say, by the want of a 

 certam amount of heat spread over a certain period of time, the 



July 1389. 



