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The holdfast fauna eonsystem of Laminaria hyperborea was tested for use as a means of environmental monitoring. Samples of about three litres of holdfast space are desirable sample sizes although one litre is the absolute minimum. Several natural environmental variables, namely fresh water, rock type, depth, exposure and sedimentation were shown to have no significant effects on holdfast fauna composition. In pollution monitoring therefore these can be disregarded as complicating factors. Gradients of heavy metals and water clarity around the U.K. were defined. To these two variables were added those of latitude and longitude, and these four variables have marked effects on holdfast faunas. Changes in holdfast composition at the 35 main sites sampled are interpreted in the light of these variables. Along the North Sea and West coast sewage pollution and heavy metals respectively have a marked effect on the fauna. Along the South coast unidentified variables correlating very closely with longitude are important. Emphasis in interpretation must be placed on numbers of organisms, species richness and diversity in known sample sizes of holdfasts, and on the community trophic structure. Species presence / absence information has no meaning in holdfast work.

A study was made of factors influencing hair development in 34 heterocystous and 2 non-heterocystous strains of Rivulariaceae. When grown with a high level of NO(_3)-N, 33 of the heterocystous strains lost their heterocysts; 19 became untapered, but I4 produced tapered trichomes which resembled those of Homoeothrix or Ammatoldea. One strain was unchanged in morphology. Hair development was reduced but not eliminated in the 3 strains with long hairs in the heterocystous state, and was unaffected in the 2 strains with short hairs. When one heterocystous strain was starved of Ng, heterocyst frequency increased, but tapering was unaffected and no hairs developed. These responses suggested that hair development and tapering in heterocystous Hivulariaceae are not necessarily directly related phenomena, and that neither is likely to be due solely to nitrogen deficiency in the apical cells. The influence of phosphate, iron, magnesium, calcium, molybdenum and sulphate deficiencies on hair development was examined, 12 of the heterocystous strains, and one Homoeothrix showed increased hair development under phosphate deficiency; 8 of these (heterocystous) strains also did so under iron deficiency, and one off the 3 did so under magnesium deficiency. The other deficiencies had no marked effect on hair formation. Several other morphological responses to nutrient deficiencies are also described. Hair development in deficient cultures preceded any degenerative changes, and did not appear to be merely a pathological symptom. The cells of hairs formed under phosphate deficiency were found to be capable of polyphosphate synthesis, indicating metabolic activity. The possibility is considered that the hairs of Rivulariaceae may have a functional significance, perhaps in the uptake of nutrients present at a low concentration. During the work some strains suddenly developed morphological abnormalities when grown in standard medium. Attempts to elucidate this phenomenon are described in the Appendix.

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