The genus Senecio in Britain contains a number of species similar in life form and morphology, which yet differ markedly in their distribution in relation to the height of the water table. This paper is concerned with the seven species which occur in Fife. Senecio squalidus, S. viscosus and S. sylvaticus are all species found exclusively in drier areas, their principal habitats being arid waste places, ash pits, railway banks and sand dunes. In all these habitats it is most unlikely that the roots of the plants will ever have to contend with a high water table. On the other hand, S. aquaticus and S. smithii are both to be found in wet ditches and loch margins where their root systems must frequently be flooded. The two other species investigated, S. jacobea and S. vulgaris, are more wide ranging in their distribution, being found both in pasture land, gardens and sand dunes. For these experiments one race of S. jacobea was used, that of a coastal sand dune population and two races of S. vulgaris, one from the sand dunes and another from a local garden. That some plants can grow unaffected by high water table conditions is usually attributed to their ability to avoid a severe oxygen deficit in their roots by the downward diffusion of oxygen from their shoots. Both Conway (1940) and Webster (1962) have shown that the root air spaces of the plants investigated had an adequate supply of oxygen for normal root growth. The majority of such investigations (James 1953) show that, on the whole, plants avoid the difficulties encountered in high water table conditions by physical means, either with well-developed aerenchyma or shallow rooting systems, rather than by combating them with physiological or metabolic adaptations. However, this does not exclude the possibility that some plants are adapted to wet conditions metabolically, or that others are excluded from them for the want of some metabolic adaptation. Rice, and the totally submerged aquatics, have to contend with a milieu in which the oxygen concentration may only be 1/30 of that of the atmosphere and even when active photosynthesis takes place, raising it up to 5 times the saturation level, the concentration attained is only 1/6 of that of the atmosphere (Stiles 1956). It is well known that some plants are physiologically adapted to withstand partially anaerobic conditions for considerable periods. Laing (1940), working on submerged rhizomes of Nuphar advenum, found ethanol accumulated in the rhizomes when the oxygen concentration was reduced to 3%/ and freshly dug up rhizomes contained ethanol even though the leaves had none. With rice, Taylor (1942) has shown that germination can continue vigorously in the complete absence of oxygen and that this is linked with the ability of the plants to respire anaerobically. One of the first signs of damage to appear in plants susceptible to flooding is a chlorosis of the leaves. It is known that an increase in the carbon dioxide concentration in the soil