SEAWEEDS 



287 



predominance of filamentous and encrusting, often 

 inconspicuous, forms, exhibiting a bluish-green 

 colour when examined under the microscope ; (6) 

 a zone from near high-water mark U> half-tide level, 

 occupied chiefly by algae of a bright grass-green 

 tint; (c) from half-tide level to low-tide murk, 

 where the majority of the plants are olive-brown 

 in colour; and finally (rf) from near low-water 

 mark to depths never exposed by the lowest ebb, 

 where the preponderance of forms are of a distinctly 

 red colour. Although not a few shores exhibit 

 these zones fairly clearly, it must l>e remembered 

 that, owing to the nature of the shore, the charac- 

 ter of the tides, and the absence or presence of large 

 influxes of fresh water, the zoning is not always 

 clearly defined. For instance, the uppermost 

 zone of blue-green forms is most likely to be met 

 with on low-lying coast lines where the seashore 

 proper merges gradually into brackish marshes and 

 muddy ]x>U, such as, for example, the estuaries 

 of the Dee and Mersey, or of the Clyde near Dum- 

 barton. Grass-green forms are most plentiful on 

 shelving rocky shores; whilst on such steep rocky 

 shores as those of the fjords of the west coast of 

 Scotland olive-brown seaweeds form the prevailing 

 vegetation right up to high-water mark. It must 

 be also borne in mind that certain species of algre 

 are invariably found in regions of the littoral area 

 not characteristically occupied by the majority of 

 forms of a similar hue. Thus the common dwarf 

 species, Pelvetiii raiialicnlata, closely allied to 

 the familiar bladder- wrack (f'tc'itx iv.v/cw/*x), 

 belongs to the olive division of seaweeds, and yet 

 is invariably found at or even above high-water 

 mark. Tangle ( /,<///'////'/< iliiiilntn), also an olive 

 seaweed, is, on the other hand, a deep water form 

 only exposed at low tides, whilst the genus 

 Struvea, one of the pure-green alg.-r, may \- 

 dredged from 30 fathoms. Very many red algii- 

 are found under cover of the olive forms lietween 

 tide marks, whilst some have their habitat at or 

 above high-water mark. For example, two species 

 of the genus Rhodochorton form a crimson velvety 

 pile on rocks from half-tide level to far al>ove high- 

 water mark, and another and easily reoogniMtMe 

 form, Catfiief/it n/mntia, grows on the lee side of 

 rocks which are seldom touched by the flood-tide. 



It is worthy of note that the clo.-si(icatiou of 

 seaweeds into four groups according to colour is 

 strikingly supported by the morphology and life- 

 history of the forms so brought together. This 

 fact becomes all the more remarkable when it is 

 rememlx'red that colour among higher plants is in 

 very tew cases of even specific value in classifica- 



tion. The blue-green algte are known as Cyano- 

 pliyce.v, the pure-green as Chlorophyceje, the olive 

 as Phseophyce;e, and the red as Rhodophyceie. 

 ( By some authors the lower members of the lirst 

 two groups are classed together under the name 

 of Protophycea?.) All possess the green colouring 

 matter chlorophyll, but in the bine, olive, and red 

 forms additional colouring matters (phycocyanin, 

 phycophiein, and phyeoerythrin) are present to a 

 greater or less extent, masking the pure-green 

 tint so well seen in the Chloropliyce.-e. It is im- 

 possible in the present state of our knowledge to 

 dogmatise on the precise value of these additional 

 pigments, but we cannot be far wrong in saying 

 that they are associated with the modification of 

 the intensity or quality of sunlight, and aid or 

 protect the chlorophyll in the peculiar and vitally 

 important duties which it performs in the nutrition 

 of the organism (see CHLOROPHYLL). It has Keen 

 ascertained that certain rays of the solar spectrum 

 are more efficient than others in the work of 

 assimilation, and it is worthy of note that these 

 rays ate precisely those which are first intercepted 

 in the passage of sunlight into sea-water. Our 

 ignorance of the important physiological problems 

 involved may be estimated when we place against 

 this explanation the fact that Kjellman in his 

 exploration of the flora of the Arctic Sea found 

 that alga' grew and reproduced at a mean tempera- 

 ture of - 1 C. and during the long and dark arctic 

 night of three months' duration. 



Taming from the bathymetric distribution of 

 seaweeds to their surface distribution, we find 

 here also many interesting find ilitliciilt problems. 

 In the lirst place the medium in which seaweeds 

 live is fit a more uniform temperature than that 

 to which land plants are exposed, although against 

 this we must place the fact that seaweeds are more 

 susceptible to fluctuations of temperature. AVhilst 

 ocean current* are undoubtedly the chief agents 

 in the transport of the marine flora, long tracts 

 of deep ocean must prove serious barriers to the 

 migration of littoral s|>ecies. It is scarcely neces- 

 sary to point out that continental areas, hotter 

 ami colder regions of the sea, and long stretches of 

 sandy shore must also act as harriers to possible 

 migration. The effects of such barriers are well 

 seen in comparing the floras of the tropical Atlantic 

 and of the Indian Ocean, the north and south 

 temperate Atlantic, and the eastern and western 

 shores of the same great ocean. From the follow- 

 ing table (abstracted from Murray's paper on the 

 ' Distribution of Marine Algne,' "Trans. JSiol. Soc. 

 Liverpool, vol. v. p. 164) it will be seen that, 



taking three principal regions into consideration, 

 the Arctic marine flora consists of 2f>9 species 

 bstonging to 111 genera, the West Indian region 

 possesses 788 species and 150 genera, the Australian 

 ll.'t'J species in 2.>" genera, and that nevertheless 

 only 17 genera and 5 species of lihodophycece, 6 

 genera anil I species of 1'ha-ophycea-, 5 genera and 

 6 species of Chlorophyce.T, and 4 genera and no 

 species of IVotophvcene are common to all. 



The structure, life-history, and classification of 

 the lower algie (lioth fresh-water and marine) 

 having been dealt with in the article Algiu (q.v.), 



there is left for treatment the two higher groups 

 viz. Phieophyccii- and Rhodophycea'. It will be 

 most in accordance with the aim of the present 

 article to sketch very briefly the main lines of 

 classification, and to select a few typical examples 

 for more detailed notice. 



The Phipophyceie include all the olive-brown 

 seaweeds found on our shores, and are subdivided 

 according to their methods of reproduction into 

 three chief series vi/.. (a) the Phirosporeie, 

 represented by such forms as Ectocarpus, Cntleria, 

 Sphacelaria, and the giants among seaweeds, 



