9 20 Proceedings of the Royal Irish Academy. 



July and August in 1911 are 58T° and 58 - 2 c . West and north of this 

 isotherm, Oaks of large size are exceedingly scarce, and although actual 

 records of individual trees may be quite misleading as climatic indices, the 

 writer cannot recall having seen any tree in the north-west of Ireland which 

 would be regarded as of average size in England, except, perhaps, on excep- 

 tionally favoured sites, as at Westport House, the neighbourhood of Sligo, 

 and other localities with good soils and shelter from westerly winds. 



In Scotland, Hutchinson' recorded oaks of over 80 feet from Eoss, Perth, 

 and Clackmannan, and Elwes and Henry 2 record an Oak of 118 feet in 

 Perthshire, the last-named county showing the greatest number of tall trees 

 north of the line referred to above. Apart from a few favoured individuals, 

 therefore, the Oak becomes a comparatively small and slow-growing species 

 in the extreme north and west of the British Isles, although few, if any, 

 parts of the latter are outside its limits of distribution in the form of scrub, 

 or fail to show its occurrence as a timber-tree in earlier times by remains 

 preserved in peat bogs. 



If this July mean of 59° Fahrenheit is necessary for the normal vegetative 

 development of the Oak, it is certainly necessary, and probably less than is 

 required, for its normal reproductive functions, which require more heat than 

 suffices for growth. The truth of this statement with regard to broad- 

 leaved species in general was well exemplified in the case of the Spanish 

 Chestnut in Ireland in 1911. This species attains as large dimensions in 

 the south and midlands of Ireland as in any part of the British Isles, but 

 the ripening of its fruit is a rare occurrence apart from the extreme 

 south-east of the country. In 1911, when the July means reached 60° 

 to 62° Fahrenheit, crops of ripe nuts were general south of Dublin, and were 

 produced in other parts of Ireland. In the case of the Oak, acorns are able to 

 ripen in all parts of Ireland in summers of average warmth, but the seed years 

 are few and far between ; the acorns are exceptionally small, and the crops 

 poor, as compared with English yields. For the Oak to have become the 

 dominant tree throughout the west of Ireland, a more plentiful seed-pro- 

 duction must be assumed than that now taking place ; and the same may be 

 said in a general way of the Hazel, although the long growing season probably 

 favour's the latter species more than the Oak. That this heavier seed- 

 production was due to greater summer warmth seems the only conclusion 

 that can be arrived at, especially when due consideration is given to the 

 fact that Oak scrub is found at much higher elevations throughout the 

 British Isles than those at which the species now attains a timber size, or 



1 Trans. Highland Agric. Soc, vol. xiii, 1SS1. 2 " Trees of Great Britain and Ireland." 



