CLIMA.TE AND TREE GROWTH 91 



of the growing season, and possibly the assimilation and 

 the formation of wood may proceed with a longer growing 

 season than that of three months. Probably the larger 

 size attained by trees growing in lower latitudes, as com- 

 pared with those growing in the north, may be due to this 

 cause. In the British Isles, however, perceptible growth 

 in late leafing forest trees can scarcely be said to begin 

 before June in an average season. With early leafers the 

 growth made in May is frequently destroyed by frost, so 

 that any gain in the period between the beginning and 

 end of growth is lost. Growth after August again is often 

 seen in the species already named, as larch, poplar, willow, 

 etc. Occasionally other species, either owing to habit, 

 accident, special weather conditions, etc., make a second 

 growth in July or August, as the oak or beech, but this 

 growth is usually completed in a month or six weeks. A 

 second growth in conifers often takes place in larch, 

 Douglas fir, pines, and spruce, but may be considered 

 more or less abnormal. 



If the three months of June, July, and August are 

 taken as the true growing season, an examination of 

 records of mean temperature show that July is almost 

 invariably the hottest mouth, or that in which the 

 monthly maxima and means culminate. So far as the 

 north-west of Europe is concerned, the amj)litude or 

 range of mean temperatures of these three months does 

 not exceed 10' F., and in most instances does not exceed 

 5" F. In all cases the mean temperature of July may be 

 regarded as an index to that of the other two months, 

 and a station showing the highest record for that month 

 may be regarded as enjoying the hottest summer. The 

 following table gives the July means for a number of 

 stations in Northern Europe, and it will be seen that 

 the coolest Julys are experienced in Scotland and Ireland, 

 and the hottest in Berlin : — 



