84 Watson's climatic zones. 



of plants are found in this zone is more a matter of soil and exposure 

 than of temperature. A large number will ascend as high as there 

 are suitable rocks, that is, usually, where there are moist ledges ; 

 their limit being due to other conditions than that of temperature 

 in this country. To say that "within certain limits they are 

 influenced by the proximity of the summit of a mountain and 

 prevented from approaching it" is misleading, unless it be taken 

 to mean that there is no suitable ground for plant-growth near the 

 summit, except for a small number of species. This is seen on Ben 

 Lawers, in Perthshire, where at 3600 ft. alt. on one side of the hill 

 there are moist ledges of rocks with many plants ; while at the 

 same elevation on other parts of the hill there is the usual gravelly 

 detritus of the tops, unsuitable except for very few species ; and it 

 is to b3 noticed that they are the species which also reach the 

 SLunmit, and are mostly plants with a considerable range of altitude, 

 as Silene acaulis, Saxifraga stellaris, Festiica ovma, and Lycopodium 

 Selago. The forty-six species given in Cijb. Brit. vol. iv. p. 323, 

 which reach 3900 ft. and above, in the "upper limits of the 

 Grampian Mountains," are almost all plants with a very con- 

 siderable range of altitude, and which do not especially affect the 

 summits. In fact, our rarest alpine plants are so often found 

 grouped together at special spots, or in some particular corrie, that 

 it would seem probable that their position is due to former geological 

 conditions of a local nature rather than to general climate. The 

 altitudinal distribution of plants in the southern part of Norway is 

 the same in general as in this country; that is to say, most of the 

 species which ascend our hills also ascend the hills in Norway, but 

 they do this usually to a greater height in the latter, though in a 

 higher latitude and with a lower average temperature. We see here 

 that, in comparison tvitk our own, the conditions of the surface, as 

 higher hills in Norway with their greater surface-room for plants, 

 affecting the altitudinal distribution more than climate does. 



The so-called "trespasses" of plants from higher to lower alti- 

 tudes at the side of waterfalls, whicli Watson considered to be due 

 to a lower temperature caused by the cold spray of the water, is, 

 I think, mamly the result of the site being favourable for plant- 

 growth in general, being moist good holding ground ; as it is in 

 such places that the lower ground plants are found to reach their 

 highest point, except in the case of a dry-soil plant as Pteris ; and 

 this would not be the case if the supposed coldness of such places 

 affected them. 



It is interesting to note, in connection with this subject, that at 

 a meeting of the Scottish Meteorological Society, in July, 1896, a 

 communication was made by Mr. R. T. Omond, of the Ben Nevis 

 Observatory, in which it was stated that the rate of change of 

 temperature for height on Ben Nevis, as on the Swiss hill stations, 

 was found to be one degree Fahrenheit for 270 ft. Watson, judging 

 from the observations known at his time, and from those made by 

 himself, gave 1° F. for 300 ft. ; but he mentioned that a difference 

 of a tenth either way, 270 ft. to 330 ft., would be less than the 

 variations at equal levels from local conditions. He inclined to the 



