The high rate of mortality in seedlings as compared with that which occurs amongst adult plants was clearly established by Salisbury (1930), and has been confirmed by numerous subsequent workers. However, whilst this particular aspect of seedling mortality has been emphasized, the equally remarkable persistency of many seedlings during this critical seedling phase has received little attention. There is evidence that the seedlings of many species have great persistency under apparently highly unfavourable conditions. Chippindale (1932) sowed Lolium italicum in boxes alone and with agriculturally useful varieties of Festuca pratensis, Phleum pratense and Poa trivialis. Lolium italicum rapidly developed a dense turf in all cases but he found that the other three species continued to survive as very small seedlings. He found that the apparently delayed establishment of these three latter species in field sowings was due to the vigorous competition of L. italicum for both light and nutrients. In a later paper, Chippindale (1948) found that seedlings of agricultural grasses were capable of persistence for several months when sown on nutrient-deficient substrata in full light, but that in the dark their persistence were temperature dependent. A. W. Davison (personal communication) showed that, both in the field and the laboratory, seedlings of Deschampsia flexuosa, Rumex acetosa and Digitalis purpurea were capable of prolonged survival on phosphorusdeficient calcareous soils. Hutchinson (unpublished data) has shown similar seedling persistence for up to 20 months in the field in seedlings of a number of species which suffered from lime-induced chlorosis and phosphorous deficiency. Grime & Jeffrey (1965) showed that several species found in woodlands were better able to survive in deep shade than were species which do not occur in woodlands. Seedlings are not commonly found in deeply shaded situations, such as occur at the base of tall grass swards, amongst the herb layer in woods, or beneath the shrubby layer of moorlands. The seedlings which do occur in such shaded situations are generally assumed to have a very limited expectancy of life. Their future development would seem to depend upon the creation of temporary gaps in the vegetation. The findings already referred to, however, suggest that dwarfed seedlings which occur in deeply shaded situations and especially on nutritionally poor soils, may be capable of prolonged survival, and of resuming vigorous growth upon the removal of the inhibitory factors. Experiments were, therefore, carried out to determine exactly how long seedlings were capable of survival in the dark, and whether any differences which occurred could be related to their normal ecological habitats. The effect of seed size, soil fertility, relative growth rate and temperature were also examined.