xii FLORA ORCADENSIS. 
rock formation underneath, do constitute conditions 
which determine slight changes. The serpentine 
rocks of Unst have Arenaria Norvegica, which 
is found nowhere else. The Ward Hill of Hoy 
possesses one species not found in the other two— 
Caithness and Shetland—viz., Dryas octopetala. Of 
the 560 odd species found in Orkney—rather more 
in Caithness and less in Shetland—there are com- 
paratively few species found in one county and not 
in the others, or in two counties and not in the third. 
I am not using the term “three counties” politically, 
but botanically. 
This is a list of plants not common to the three 
counties, prepared by Mr Arthur Bennett, F.L.S., and 
published in “Annals of Scottish Natural History,” 
January 1909 :— 
NAME OF PLANT. Orkney. Caithness. Shetland. 
Avabis petra ..., sa. os. see No... | @lipeae ee 
Subularia aquatica... ... ... No ..t Yess eee 
Sagina subulata ... ... ... No « “Bip 
Cerastium Edmonstonii... ... No .... UNG Seales 
Arénaria rubella .:. ... No .... ipsa eine 
Hypericum quadranulum _... No. 1. “¥Ssge ene 
Geranium sylvaticum ... ... Yes ..3,. Meae .7a Nia 
Rosa mollis (G5 a. de Yes °<.. Sigs ees ee 
Alchemilla filicaulus ... ... Yes... JN ile ee 
Sibbaldia procumbens ... ... No. get GND Seen ore 
Dryas octopetala ... ... Yes goss | OR Loree Blue 
Saxifraga stellaris ... ... ... Yes ‘ixc M68 Song oe 
Callitriche hamulata ... ... Yes ‘3:5 Bear) var Ne 
Callitriche stagnalis ... ... Yes wa (Mes )../ ia 
Epilobium alsinifolium ... ... No j.0-) Bile. Ogee 
Garnns BUECICA:. i nn.) ) wes, een Yes 2: ) No Stee 
Matricaria pheeocephala... ... No. 3s Wiese eee 
Taraxicum spectabale ... ... No... 0 fone 
