106 ANNALS OF SCOTTISH NATURAL HISTORY 



ON SCOTTISH 



By JOHN ROY, LL.D. 



IN the following pages an attempt is made to bring the 

 knowledge of " Scottish Desmidieae " up to date, and to 

 indicate their distribution throughout the country. From 

 their very nature, both subjects must necessarily be imperfect. 

 The northern and midland counties have been fairly well 

 examined, some of them very well, but we have almost no 

 information from south of the Firths of Forth and Clyde, 

 except from Renfrew, Bute and Arran, and Kirkcudbright. 



No attempt has been made to give the distribution of 

 species in altitude. This cause does not appear to exercise 

 any great influence on their distribution, most species seem- 

 ing to be found both at high and low altitudes. A very 

 few species seem to cling to marshes formed by melting 

 snow, where it lies late into the summer and autumn, from 

 3500 feet upwards. Cosmarium nasutum, Nord., and Stanr- 

 astrum Kjelhnanii, Wille, may be cited as typical examples 

 of these. 



Perhaps the most marked influence in determining the 

 number of species in a district is its geological formation. 

 Granite appears to be the most favourable, limestone less 

 so, and sandstone very poor. Hence the basin of the Dee, 

 in Aberdeen and Kincardine, which is almost wholly granite, 

 has turned out well, one marsh alone yielding about 300 

 species ; while in Strathmore, on sandstone, it is seldom 

 possible to get more than forty or fifty species in a gather- 

 ing, and usually not nearly so many. Sometimes, however, 

 in these scanty gatherings, a little patience reveals a rarity : 

 e.g. Cosmariwn biretnm, Breb., van supernumeraria, Nord., at 

 Keithick, near Cupar Angus ; and Cosmarium siibortogomiin, 

 Raciborski, at Ballendoch, near Alyth. The direction of 

 the watershed of a country has also something to do with 

 distribution, though probably not so much as was at one time 

 supposed, and certainly not nearly so much as it has in the 

 case of flowering plants. In Scotland the chief watershed 

 runs from north to south, giving rise to an Atlantic and 

 Germanic slope. Almost the only species known at present 



