236 BAKER*S NORTH YORKSHIRE. 



heat being intense enough, and the period of time during which 

 it is continued long enough, to enable them to produce their 

 flowers and ripen their seeds, in the case of perennial species, if 

 not every year, at least occasionally. This negative restriction 

 must be the main one, and will apply to annuals, biennials and 

 perennials alike, although the different periods of the year at 

 which different species develop themselves must make a con- 

 siderable difference in its application. Limitation by excess Oi 

 heat often shews itself as limitation by the lessening of humidity, 

 but with us, in restricting the distribution of plants, it evidently 

 operates only in a comparatively unimportant manner. 



The sums of summer heat and the extreme minima of the 

 colder parts of the year are then the data of temperature with 

 which botanical geography is specially concerned. The following 

 propositions embody the principal details with regard to the 

 distribution of our local temperatures which it seems needful to 

 recall to mind in this connection. We must remember that in 

 observations upon temperature it is the monthly means of the 

 air in the shade that are stated, and no doubt these upon the 

 whole are the data which are most valuable to have. But they 

 are deduced from the average of a number of years, and some- 

 times the temperature of a month rises above and sometimes 

 falls below the average. They take equal cognizance of all 

 degrees of temperature alike, both those which affect plants and 

 those which do not. The temperature which a plant receives 

 is partly that of the air, and partly that of the ground, which is 

 somewhat different to, and much more uniform than, that of the 

 air. There is a wide difference, especially at the warmest part of 

 the year, between the temperature in the shade and in places 

 which are exposed to the sun, as the tables which are given at 

 page 88 shew in detail ; and what has been stated respecting the 

 temperature of springs may also suitably be referred to in con- 

 nection with plants. These propositions embody those details 

 with regard to the distribution of local temperature which seem 

 to have any prominent bearing upon the question we are now 



