. Elements of ecology. Ecology. 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 —°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. ). + 10. 3 2 1 0-1-2-3-4
. Elements of ecology. Ecology. 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 —°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. ). + 10. 3 2 1 0-1-2-3-4 -5 -6 -7 Average temperature for January,°C Iversen, 1944 Fig. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Clarke, George L. (George Leonard), 1905-. New York, Wiley
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