Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/126

100 can go a step further, without risk of error, if we consider that it is a treeless flora, which will therefore occur outside of the limit of trees of that time. It is not necessary, of course, that the former limit of trees had the same temperature as that of to-day. The present limit of trees coincides with the 10° C. isotherm of the warmest months with an accuracy which at first sight is amazing. This is easily explained by the fact that trees, by their height above the ground, are dependent on the temperature of the air which we can measure meteorologically, whilst the tundra-flora,

clinging as it does to the earth, makes use of the greater warmth of the soil and the atmosphere lying immediately above, heated by the uninterrupted day and night rays of the sun. Thus it secures a longer period of growth with temperatures of 10° or upwards right up to the poles. The Carboniferous limit of trees will have played a similar rôle even if it is possible that there was another limiting temperature for the