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The percentage of clover in a mixture of Huia white clover and Mellc perennial ryegrass decreased during each of three winters. In the most severe winter, both grass and clover suffered a net loss of weight, clover losing twothirds of its maximum weight while grass lost about one-third. In the mildest winter, grass continued to gain weight throughout, while clover lost weight. Such losses, when combined with conditions such as the use of nitrogen fertilizer, which prevent a recovery of clover percentage during summer, will lead to a longterm decrease in the clover content of a sward.The losses from the clover were contributed to by all plant parts especially leaf, which lost between 60% and 95'^i) of lamina weight. The change from autumn to winter caused a greater decrease in the number of live leaves per shoot, and In the area and weight of individual leaves, in clover than in grass. Other work has shown that there is a greater decrease in photosynthesis in clover than in grass, probably owing to changes in canopy structure; this is one cause of clover's poor performance, though differences between the two species in the effect of winter on the distribution of the carbon Tixed in photosynthesis and in losses of weight in respiration and tissue death may also be involved.

In two experiments with ryegrass/white clover mixtures, the proportion of clover was measured before and after cuts which removed 8-75% of above-ground biomass. Cutting was found to reduce the proportion of clover leaf area in the crop in both experiments, sometimes by as much as two-thirds, and the proportion of clover dry weight, by up to half, in one of them. That is, the harvested material contained a greater proportion of clover than did the sward before the cut. This disproportionate removal of clover was due to clover having a greater proportion of its leaf near the top of the canopy than grass. It showed that preferential removal of clover occurs as a result of the purely passive selection by a mower, not only as a result of grazing by animals which may be capable of active as well as passive selection.Despite the disadvantage to clover of losing more of its leaf area than grass, and in some cases more of its dry weight also, when the mixture was cut, the clover content of the sward did not decrease during the growing season as a whole. This was because, where no nitrogen fertilizer was applied, clover had a greater relative growth rate (RGR) than its companion grass during the growth periods between cuts and this increased its percentage of the mixture.

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SUMMARYMeasurements of canopy gas exchange of mixed communities of ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) in the field during winter showed that instantaneous rates of gross photosynthesis were usually little less tlian those of canopies of similar leaf area at similar irradiance in summer. Respiration rates per unit dry weight were on average 25 % less than those of comparable swards in summer. The daily totals of net photosynthesis, however, were as little as a tenth of summer values, because of the short photoperiods and lowaverage irradiance. Clover leaf area tended to be positioned lower in the canopy than grass leaf area, especially towards the end of winter, and clover had a lower rate of photosynthesis per unit leaf area in situ than grass.