552 



tive plants, however, it is not confined to one season, but continues 

 to develop throughout the year; but with this difference, that the 

 plants developed in summer are very much smaller, more tenacious, 

 and of a dull colour. These last are regarded by some authors as a 

 different species, and called P. umbilicata. 



" There is a circumstance connected with the history of our com- 

 mon Ulvse, Enteromorphse, and Porphyrse, which deserves notice. 

 Most of the species common to the European shores are found in all 

 parts of the world to which a marine vegetation extends. In the cold 

 waters of the Arctic sea, Ulva latissima, Enteromoipha compressa, 

 and Porphyra laciniata vegetate in abundance ; and these same plants 

 skirt the shores of tropical seas, and extend into the southern ocean 

 as far as Cape Horn. Vegetation, at least with its most obvious fea- 

 tures, ceases in the south at a much lower parallel than in the Arctic 

 regions, and the shores of the Antarctic lands appear to be perfectly 

 barren, producing not even an Ulva. But the fact of the great 

 adaptability of plants of this family to different climates, is beauti- 

 fully illustrated by the last land plant collected by the acute naturalist 

 attached to our Antarctic expedition. The last plant that struggles 

 with perpetual winter was gathered at Cockburn Island, 64° S. (a la- 

 titude no greater than that of Archangel, where the vine is said to 

 ripen in the open air), and this proved to be an Ulva (U. crispa*), 

 identical with a small species which may often be seen in this country 

 on old thatch, or on damp walls and rocks, forming extensive patches 

 of small green leaves. It is not common to find marine plants with 

 so wide a distribution ; but a nearly equal extent of sea is charac- 

 terized by another of the British Chlorosperms, of a much greater 

 size and more complex structure. On most of the rocky coasts of 

 Britain may be gathered, in tide-pools, or rocks near low-water mark, 

 an Alga of a bright green colour and spongy texture, cylindrical, and 

 much branched, the branches dividing pretty regularly by repeated 

 forkings, and the whole invested, when seen under water, with a downy 

 coat of colourless filaments. The name of this plant is Codium to- 

 mentosum. Under the microscope it is found to be wholly composed 

 of small threads, of a tenacious, membranous consistence, filled with 

 a dense granular fluid, closely and intricately matted together ; the 

 threads in the centre of the branches having a longitudinal direction, 

 while those of the circumference are horizontal, presenting their 



" * See ' Flora Antarctica,' vol. ii. p. 498. In the northern hemisphere, Ulva 

 crispa extends to Spitzbergen, in lat. 80°. 



