|Résumé en anglais
||A study of L. digitata was carried out over the period 1965 to 1970. Monthly examination of marked plants showed that there was no single period of growth during the yr although in the third year a phase of activity giving rise to an increase in the size of the lamina and fronds was evident. Ageing of the alga was associated with a progressive reduction in meristematic tissue. A study of a `deep' population, never uncovered by water, was also carried out and although these showed similarities to the populations on the shore, there were quantitative differences, the size of plants in the deep population being very large. The effects of temperature and light on the alga's growth were examined in an attempt to determine the cause of this inequality in development between the 2 populations and it was found that the particular spectrum of light had the greatest effect on spore germination, development of the prothallus and plant growth. Meristematic activity was assessed by the lamina's propensity for regeneration. 6 to 10 months after cutting this away regeneration was complete, the measurements of the newly formed lamina being generally greater than the original. Regeneration of the fronds was possible in the first year but the capacity was lost when the plant was 4 yr old. The importance of this capacity for regeneration in repopulation of beds is pointed out. In view of the importance and exploitation of alginates a study was made of the levels of alginic acid. There was considerable variation, levels being greatest at the growing point, diminishing with elongation and being low in the reproductive regions. Levels were also lower in old plants, the maximum amounts being found in plants 3 year old. When comparisons were made between different populations, it was found that the greatest amounts were present in plants growing at a depth of 4 m. The degree of polymerisation of the algines was assessed by measuring the viscosity of a 1% solution of sodium alginate and the results show that maximum polymerisation occurred during the winter months. The degree of polymerisation was also found to fall with age. The observations made do not suggest that the coasts are likely to be depopulated through exploitation for algines; the meristematic and reproductive capacity of L. digitata is so far well able to cope with repeated cropping.