DISTRIBUTION OF PLANTS IN BRITAIN. 715 



are confined to the northern parts of Great Britain, while the species 

 of Cystoseira, Pucus tuberculatus, Halyseris polypodioides, Rhody- 

 menia jubata, R. Teedii, Microcladia glandulosa, Rhodomela pinas- 

 troides, Laurencia tenuissima, Iridsea reniformis, and many others, 

 are confined to the southern parts. The proportion of the different 

 marine plants on the shores of Britain are as follows : — Melanospermeee 

 l-5th, Rhodospermese 3-8ths, and Ohlorospermeae l-4th of the whole. 



Dickie, in speaking of the British Algae which have a southern 

 type, says that they may be classed under three heads — 1. Those 

 confined to the southern parts of Great Britain and Ireland ; 2. Species 

 of a more extensive range, which extend to the north of Ireland and 

 south-west of Scotland ; and 3. Those found abundantly in the south 

 of England, and ranging along the western coasts of both islands as 

 far as Orkney and Shetland. The species comprehended under these 

 three heads, and amounting to at least 20, seem to be absent from a 

 certain part of the east coast of Scotland. A considerable portion of 

 them re-appear in Shetland and Orkney. He thinks that the appear- 

 ance of southern forms of Algse, at the extreme northern parts, is to 

 be attributed to the influence of the Gulf Stream as regards tempera- 

 ture. 



British Algse are variously distributed, some in deep, others in 

 shallow water. Laminaria digitata only extends to the low line of 

 ebb during stream tides ; L. saccharina flourishes along an inner belt, 

 partially uncovered during the ebbs of the larger neaps ; Fucus ser- 

 ratus and P. nodosus thrive in a zone still less deeply covered by 

 water, and which even the lower neaps expose ; F. vesiculosus occurs 

 in a zone higher still, altering its form as it goes farther inland ; F. 

 canaliculatus also rises high on rocky beaches. If land-springs escape 

 from the beach there may be found an upper terminal zone of Con- 

 fervse mixed with Ulva latissima, Porphyra laciniata, and Entero- 

 morpha compressa. In the lake of Stennis at Stromness, Orkney, 

 there occur at the part where the sea enters, specimens of Fucus 

 nodosus and vesiculosus in their ordinary form, along with Halidrys 

 siliquosa. A little farther in, where there is more fresh water, 

 Halidrys and Pucus nodosus disappear ; P. vesiculosus becomes 

 stunted, its air-bladders being altered or disappearing ; and ultimately 

 it becomes narrowed like the Confervse, and altogether loses its usual 

 aspect. 



The British marine plants, according to Forbes, are distributed in 

 depth or bathymetrically in a series of zones or regions which extend 

 from high-water mark down to the greatest explored depths. The 

 first or littoral zone is that tract which lies between high and low 

 water marks, and therefore is very variable in extent according to the 

 amount of rise and fall of the tides. It has been divided into sub- 

 regions characterised by the prevalence of certain marine species. 



