398 Dr Christison on the [se 



SESS. LIII. 



of girth-increase in 1879 was attributed by Sir Eobert Chris- 

 tison to three climatic causes — a great reduction of the mean 

 temperature of every gi-owing mouth, a similar reduction of 

 sunshine, and as a result of both these climatic causes, the 

 loss of June as a growing month. Of low winter tempera- 

 tures he makes no mention, but perhaps their influence ought 

 not to be entirely ignored, as it appears from Mr Buchan's 

 returns that the mean temperature of December was only 

 31°, and that the minimum was so low as 9°. The result 

 in this first year of depression was that both the deciduous 

 and evergreen classes suffered a heavy loss of girth-increase, 

 the former falling from llj to 8 inches, and the latter from 

 8 1 to 6 inches. 



The depression of 1880 was attributed by Sir Robert to 

 the extraordmary low temperatures of the previous Decem- 

 ber, succeeding an autumn unfavourable to the ripening of 

 wood and formation of buds. The lowest temperature of 

 December at Edinburgh, as recorded in Mr Buchan's tables, 

 was 7°'o ; and' the unprotected thermometer at the Botanic 

 Garden gave the low temperatures of 1°, 4°, 13°, and 17° 

 on different nights. Great damage to trees took place 

 throughout Scotland in this season, and in the measured 

 deciduous trees, as a class, the annual girth-increase fell to 

 its decennial mininmm, that is to say, from the maximum 

 Hi inclies in 1878 to 6 inches in 1880. The evergreens, 

 as a class, however, fell no lower than in 1879. 



The following winter was even more trying, at least in 

 the Botanic Garden. The greatest cold fell in January, 

 when the minimum recorded in Mr Buclian's tables was 9°-4 ; 

 but the unprotected instrument at the Garden fell to 0°, 4°, 

 7°, 10°, 11°, 12°, 12°, 12°, 13°, and 14° in different periods of 

 twenty-four hours. Unusual cold was also prolonged far into 

 spring. Nevertheless, the deciduous gToup as a class, instead 

 of suffering a furtlier fall, rallied to the highest point attained 

 after 1878, or from 6 to 9 inches, while the evergreen class 

 fell a little below even the low level of the previous years. 

 The prosperity of the deciduous class may have been due to 

 perfect ripening of the wood in the previous favourable 

 autumn, while the increase of the evergreens may have been 

 kept down by the circumstance that, as my observations on 

 monthly growth show, this class makes an earlier start in 



