- WETGHTXCr MOORTXOS 30 



WEIGHINa MOORINGS. 



Br A. H. Chitrch. 



From the stindpoint that the algae constituting the phytobenthon 

 of the sea may be preferably regarded as anchored, (hormon), the 

 problem of the security of the moorings becomes one of primary 

 significance, and conditions clearly vary within wide limits according 

 as the bottom consists of mud, sand, shingle, or clean rock. The fact 

 that the larger algaj require good holding-ground of rock, while 

 sandy coasts are comparatively bare of all vegetation, is sufficiently 

 obvious ; and it has been pointed out elsewhere that this has had a 

 remarkable influence on the history of algology, more particularly in 

 countries where the coast is predominantly of sand-dune formation. 

 The case of mud is more satisfactory, since, owing to the cohesive 

 nature of its slimy texture and the effect of bacterial zooglcea in 

 binding the surface-film, the substratum may be able to bear con- 

 siderable movement of the superjacent medium, while algae with 

 rhizoid attachment may penetrate considerable depths and assist in 

 binding the surface to constitute good anchorage for plants of con- 

 siderable size, as Chorda, 10 ft. or more, in close association. A 

 mudd}^ bottom may thus carry a distinctive flora when a sandy shore, 

 owing to the readiness with which the particles are lifted by surge- 

 action of the water, affords no secm-ity at all, and may present no 

 characteristic plants beyond loose-h'ing calcified Litlioth amnion, 

 which are practically pebbles. As the specific gravity of such sand- 

 j^articles may be taken as little over 2, it may be noted that a stone 

 in sea-water loses nearly half its weight, while irregular shapes 

 offering considerable " form-resistance " may considerably delay the 

 rate of sinking. The surging action of the waves, as an upward 

 thrust, may thus if sufficiently violent maintain in suspension par- 

 ticles of considerable size ; and the scour of the sea-shore by particles 

 and pebbles so lifted, is in fact the commonplace of the sea, and con- 

 stitutes one of the factors limiting plant-life on " exposed-coast " ; 

 but it also expresses the insecurity of the moorings of smaller algas in 

 such biological stations. A further means of moving particles of 

 even considerable size is noted in the evolution of bubbles of photo- 

 synthetic oxygen which are so extensively utilized for the erection of 

 axes, as in rounded types of lacunar and hollow thallus, or the differen- 

 tiation of special members, pneumatoj^hores with pneumatocysts 

 {AscopJiyllum, Sargassum, Macrocystis), culminating in the 6 ft. 

 bladder of Nereocystis. Since the pull of such erecting bubbles con- 

 stitutes a further strain on the hapteron- system, such forces may 

 combine to exert a considerable lifting effect on the substratum ; and 

 where the holding is insecure the plant-soma may be lifted off its 

 bed, thus weighing its moorings, to be drifted out to sea. or in shore, 

 according to the direction of the tide or current-flow. 



Thus Professor Oliver, for Blakeney (1912, in lecf.), has described 

 the germinating zoospores of Unteromorplia on exposed wet sand, 

 actively photosvnthetic and attached to individual sand-particles of 

 •25-*3 mm. diam., floated off by the incoming tide, each supported by 

 its bubble of oxygen. The most striking example of the effect of 



