Seeds of various species were sown on the exposed surfaces of sintered glass plates arranged to supply water under controlled tensions. Germination of many species was markedly influenced by tensions between 0 and 200 cm of water. Mucilaginous seeds showed least sensitivity to tension and spiny and reticulate seeds were particularly sensitive. Seeds with smooth testas showed a graded response to tension, small seeds being less and large seeds more sensitive. Seeds sown on slate dusts at controlled water tension were influenced in germination by tension (at constant water content) and by water content (at constant tension). The dependence of germination on applied water tension was greatly reduced when the sinter or slate dust surfaces were protected from evaporation. The results of these experiments are interpreted in terms of the area of contact made between seed and substrate, and the degree of exposure of the seed to the atmosphere. A heat flow analogy is discussed. The significance of these findings is discussed in relation to the interpretation of seed size and the establishment of plants from seed under natural conditions.