Contained within the 4th Edition (1974) of the Atlas of Canada is a set of three maps relating the effects and dependency of vegetation on environmental variables. One map shows the average annual potential evapotranspiration in inches from ground and plant surfaces for areas where there is a continuous vegetation cover and sufficient soil moisture for plant use. The second map shows the average annual water deficit in inches where, during parts of the growing season, soil moisture is insufficient to supply enough moisture for theoretically full plant growth. The last map shows the accumulated number of degree-days. For the purpose of this map degree-days are calculated from the base temperature of 42 degrees F/5.6 degrees C. The growing season is defined on this map as the number of days with an average temperature over 42 degrees F/5.6 degrees C.