SUMMARYNitrogen fixation has been demonstrated to occur in the cephalodia of the lichen Peltigera aphthosa which contain a species of Nostoc. The rate of fixation is very high in comparison with the maximum rates for A'. muscorum under optimum culture conditions. Virtually all of the nitrogen fixed is secreted to the lichen thallus at a rate equal to the rate of fixation. The possible control mechanisms of this secretion are discussed.

The nitrogenase activity (potential nitrogen fixation) of a mixed white clover-grass sward growing on a range of soils containing up to 21 6 pg g-' Cd, 30 000 pg g-' Pb and 20 000 pg g-' Zn was determined using the acetylene-reduction assay. The plants were incubated in situ using an intact soil-core technique. Little change in the rate of CzH2 reduction was observed during the daylight hours although a marked seasonal fluctuation was found, the maximum activity during the spring and declining to 20% of this by the autumn. Some reductions of nodule and plant size, and in nitrogenase activity, was observed in the most heavily contaminated sites. These effects were small; the plants and nodules otherwise appeared healthy. The potential for nitrogen fixation was equally high in all the sites at up to 80 g N ha-1 h-I in the spring.

Acetylene reduction by non-symbiotic, heterotrophic micro-organisms in a range of soils containing different concentrations of heavy metals was determined using intact soil cores. The suitability of this method for the soils used in this investigation was established. Samples were collected seasonally, and were incubated under standard conditions (darkness: 15%). Mean values of metal concentrations in the soil (pg g-I)were: Cd: 1-200; Pb: 60-8000; Zn: 70-26000, Cu: 20-40. Rates of acetylene reduction were generally low, from 2800 to 50 OOO nmol C,H, m-2 day-l. Assuming a 3 : 1 ratio ofC,H, reduction to N, fixation, this represents a rate of0.3 to 5.0 g N fixed ha-1 day-1 in the surface I50 mm of soil. No consistent effect of heavy metal concentration was found. The most important factors determining activity were soil moisture content and possibly inorganic nitrogen concentration. It thus appears that the bacteria in polluted soils are capable of adapting to potentially toxic concentrations of heavy metals, or that these metals are present in the soils tested in unavailable or non-toxic forms.

SUMMARYThalli oi Peltigera membranacea, Peltigera polydactyla and Lobaria pulmonaria were labelled witĥ *N and subjected to simulated episodes of heavy rainfall after periods of drying in a lighted air-conditioned incubator. Inorganic and total nitrogen was estimated in the eluates. The drying-wetting cycle was repeated three times. From the level of ^^N labelling of the eluates, it was evident that recently fixed nitrogen was released, the inorganic fraction (NH4"^) being up to three times more heavily labelled than the organic. The amounts of nitrogen lost in successive episodes generally diminished in logarithmic fashion, the level of labelling remaining approximately constant, implying little or no replenishment of mobile material from insoluble reserves. Up to three times the total nitrogen fixed per day could be lost in one rainfall episode. A preliminary trial showed that nitrogen losses from thalli under less rigorous regimes could be of nutritional benefit to the underlying moss substratum.

1. The digestive juice of the snail Helix pomatia was used in the study of the degradation of isolated cell-wall preparations from a strain of Saccharomyces carlsbergensis. 2. The crude enzyme system was fractionated by gel filtration and the activities of the more specific fractions thus obtained were examined. 3. Results are discussed with respect to (a) the nature of various factors that are essential for protoplast formation and cell-wall dissolution and (b) structures envisaged in yeast cell walls that are responsible for the observed variations in susceptibility to attack by snail juice.