50 THE CLIMATAL DISTRIBUTION OF BRITISH PLANTS. 
Similar observations made on other mountains in different 
parts of the country would again show like results; the 
plants found will differ in some respects in each case, and, 
probably, the heights at which they are found, will also 
differ to some extent, but the order with which certain 
plants disappear and others take their places will be the 
same. 
The explanation of these facts is, of course, one of 
temperature. Everyone is aware that temperature is greatly 
affected by height above sea level; that, as we ascend a 
mountain, the air gradually becomes sharper and more 
bracing, and that snow lies longer on the hills than it does 
in the valleys. 
The temperatures governing plant range are, says Dr. 
F. Arnold Lees in the able introduction to his ‘Flora of 
West Yorkshire,” not so much the annual means as the sums 
of summer heat and winter cold. Two regions may have 
the same annual mean, yet one may have a very hot 
summer and a very severe winter, while the other, like our 
climate, may have cool summers and mild winters. 
Annuals and Perennials are obviously very differently 
affected by extremes of heat and cold; the former complete 
their whole cycle of life during one year, and, as seeds will 
bear almost any cold to which they are likely to be exposed, 
the severity of the winter will have no effect upon them, 
but they will flourish if only the summer heat be sufficient 
to enable them to produce seed. Perennials, on the other 
hand, live on from year to year, and so have to bear all 
extremes of winter frost, while, on the other hand, a hot 
summer every year is of less importance to them, as, if a 
plant which lives in its mature state through ten years is 
unable during nine of them to mature its seed, the tenth 
being sufficiently warm may enable it to leave descendants. 
The manner in which cold acts in modifying the flora of a 
district, is by limiting the amount and checking the intensity 
