UNTIL the mid-seventeenth century, there were few who did not have a general belief in the spontaneous generation of life. This was a theory held particularly in the case of insects and weeds, in the days before the invention of the microscope. Insects were thought either to breed from decaying matter, or arise from 'dew' on leaves-an insect egg could in fact look very like a dew drop. Weeds were considered to generate from the very soil itself, not necessarily from parent plants. It was a general belief that any efforts to remove them were doomed to failure, for the soil being cursed at the Fall of Man would always produce weeds as a punishment.' Folk terms reflect this belief in such names as 'hellweed' or 'devil's guts' for the common dodder (Cuscuta epithymum) and 'ground glutton' for groundsel (Senecio vulgaris). There was considerable controversy also over the nature and spread of infection and disease which was not really settled until the time of Pasteur. The transfer of disease from person to person or plant to plant was invariably considered as a process of sympathetic magic. By the end of the seventeenth century, however, there was a great deal of biological curiosity abroad, and in Great Britain and Europe several learned societies had been set up to discuss and investigate new scientific ideas.2 Research had started upon the nature of insect pests, but plant diseases presented more problems and were not really understood until the nineteenth century. Then as now, weeds were one of the greatest pests affecting agricultural crops. The weed population was much higher before 1700 than was the case later, for sowing and weeding were all done by hand. In his Boke of Husbandry,3 Fitzherbert enumerated a number of weeds that 'doe moche harme', from which one may infer that these were some of the commonest types prevalent over four hundred years ago. Of these, thistles, docks, nettles and charlock are still with us, although charlock because of herbicides has ceased to be such a menace. However, others that he mentions, such as 'cocle' (corncockle), 'Darnod' (darnel) and dodder, we would be unlikely to see now; the same with 'Gouldes' or 'Guildes', the corn marigold (Chrysanthemum segetum). Owing to the primitive methods of winnowing, grains of darnel, other weeds and wheat were all liable to be harvested together; the more so because the wheat varieties then grown had grains which approximated more closely to those of weeds than is the case now. Hence such screening methods as then practised were inadequate to separate them. Shakespeare's reference to darnel in Henry VI: 'Want ye corn for bread? 'tis full of Darnel; do ye like the taste?' was no doubt quite topical. Darnel seeds are toxic and when ground with wheat produce a flour that can cause serious illness to those eating bread made from it. Like darnel, the purple flowering corncockle (Agrostemma githago) was another menace to crops-its seeds also contained poisonous glucosides when ground into flour. Gerard4 commented that 'what hurt it doth among come, the spoil unto bread, as well in colour, taste, and unholsomnes, is better known than desired'. The corn marigold was also a serious pest, so much so that in the reign of Henry II an ordinance was issued against the 'Guilde Weed' perhaps the earliest recorded enactment requiring the destruction of a pernicious weed. The blue cornflower (Centaurea cyanus) was a common pest in