68 FORESTRY OF NORWAY. 
the summer, and the average greatest heat of the summer, 
are not the same thing ; the average cold of winter, and 
the average greatest cold of winter, may be very different ; 
and the greatest degree of heat, or the greatest degree 
of cold, is not indicated by either the average greatest 
heat or the average greatest cold. A very much higher 
temperature than the average may be necessary to the 
maturing of fruit or seed, and the occasional occurrence of 
of a degree of cold, considerably in excess of the average 
degree of-cold, may prove fatal to plants which could 
have withstood even a lower average cold, without 
detriment. 
It may be accepted as a general statement that heat 
promotes vegetation, and that cold itself, a negative thing, 
operates negatively in checking it, inasmuch as it is an 
absence of the heat requisite for vigorous vegetation ; 
but greater cold, amounting to frost, may prove positively 
destructive by freezing the sap, which in its expansion 
bursts the sap vessels, as the expansion of water freezing 
in pipes bursts these pipes. A heat in excess of the range 
of temperature within which a tree flourishes may limit 
its reproduction by seed by causing it to go all to wood ; 
and a degree of cold in excess of that range may limit its 
capabilities of bringing its seed to maturity, producing, 
it may be, only a stunted growth; while an excess of 
cold beyond what would be followed by such a result may, 
as has been stated, kill the tree by the rupture of sap 
vessels through the expansion of the sap in freezing, 
Dr Broch has collected details of observations of the 
temperature of Norway in respect of all of these particulars. 
He states that it is only of late years that observations 
such as are desired have been made. There exist for 
certain places a complete, long-continued series of obser- 
vations. But these stations were limited in number; 
and though sufficient for the localities at which they were 
made, and as such valuable, the geographical distribution 
of these stations was not the one most favourable .for 
