162 Temperature 



more species when they have been more closely investigated. Ac- 

 cording to Iversen (1944), for example, the northern and eastern 

 limit in Europe for the growth and reproduction of the ivy, Hedera 

 helix arhorea, is determined by the minimum winter temperature in 

 situations where the average temperature for the coldest month is 

 — 1.5°C or below, but in areas with less severe winters the northern 

 limit is fixed by the occurrence of sufficient summer heat as indicated 

 by the average temperature for the warmest month (Fig. 5.13). 



+ 10 



3 2 1 0-1-2-3-4 -5 -6 -7 

 Average temperature for January, °C 



Iversen, 1944 



Fig. 5.13. Temperature conditions at localities in Europe in which the occurrence 

 and condition of the ivy, Hedera helix i. arhorea, have been investigated. 



• Ivy sets fruit regularly. 



O Ivy sets fruit occasionally, 



o Ivy occurs but does not set fruit. 



X Ivy does not occur. 



The harmful action of high temperature is not necessarily limited 

 to midday hours in summer nor that of low temperature to the coldest 

 night in winter. We have seen in the previous chapter that winter 

 killing of vascular plants is sometimes due to the excessively warm, 

 dry wind during periods when the ground is frozen. For many plants 

 the danger of injurious low temperatures occurs in the spring after 



