714 GEOGRAPHICAL BOTANY. 



II. Arctic Eegion — characterised by the absence of Com cultivation. 



1. Infer-arctic Zone — ^this has its terminal line at the limit of Erica 



Tetralix. 



2. Mid-arctic Zone — space ahoTe the limit of Erica Tetralix, and 



■within or helow that of Calluna vulgaris. In this zone most of 

 the rare alpine plants are found, such as Saxifraga nivalis, 

 Gentiana nivalis, Erigeron alpinus, Astragalus alpinus, Veronica 

 alpina, Alopecurus alpinus, etc. 



3. Super-arctic Zone— ahoYi the limit of Calluna, characterised hy 



Saxifraga cemua and rivularis, and Luzula arcuata. 



These six climatic zones are thus presented in a tabular form : — 

 I. Ageaeian Region. 



1. Infer-agrarian Zone — Clematis, Eubia, Cyperus longus. 



2. Mid-agra/rian Zone — Ehamnus catharticus without Clematis. 



3. Super-agrarian Zone — Pteris aqnilina without Rhamnus, etc. 



II. Aectic Eegion. 



4. Infer-arctic Zone — Erica Tetralix without Pteris. 



5. Mid-a/rctic Zone — CaUima vulgaris without Erica. 



6. Super-arctic Zone — Salix herhacea without Calluna. 



The Marine Ploea of Britain, with the exception of such plants 

 as Zostera, Zannichellia, and Naias, is Crjrptogamic, and does not pre- 

 sent very definite zones of distribution. Cryptogamic plants in general 

 can endure great vicissitudes of climatal conditions. Species of Ulva, 

 Enteromorpha, and other genera, seem to be universally distributed 

 from pole to pole. There are, however, Algse of a higher type which 

 are more limited, and the diffusion of which is determined by lines 

 of coast and depth of water. British marine vegetation presents two 

 weU-marked types according to Forbes, a southern and a northern. 

 The genera Padina and Halyseris have their northern limit on the 

 south coast of England, where they are rare. The genera Cystoseira, 

 Sporochnus, Outleria, and certain species of Sphacelaria, Mesogloia, 

 Rhodymenia, Gigartina, and Dictyota, mark out a southern region, 

 including the British Channel and part of the east coast, the Bristol 

 Channel, and the south and west of Ireland ; while the presence 

 of Odonthalia dentata, Rhodymenia cristata, E. lycopodioides, and 

 Fucus Mackaii, characterise a northern flora, on the coasts of Scot- 

 land, the north of England and of Ireland. 



On the shores of Britain, Dr. G-reville remarks, it is easy to per- 

 ceive that some species, as Gelidium corneum, Phyllophora rubens, 

 and Sphserococcus coronopifolius, become more plentiful and more 

 luxuriant as we travel from north to south ; and, on the other hand, 

 that Ptilota plumosa, Rhodomela lycopodioides, Rhodymenia soboli-' 

 fera, and several others, occur more frequently and in a finer state as 

 we approach the north. Odonthalia dentata and Rhodymenia cristata 



