The main difference between animals and plants is the larger range of sensitivities to radiation observed in plants. Different species may differ in their sensitivities by a factor of 500; if algae are included the variation may be 5,000 fold. Moreover, a given species may itself have a wide range of sensitivities, up to a factor of 50, depending on the different stages of growth. Apart from the dose itself, the effect of radiation on plants may be influenced by many environmental factors. In the case of fall-out, and additional factor is the season of its occurrence; for example, food crops irradiated in the seedling stage will be exposed for a longer time and will therefore receive a larger dose of radiation than if the fall-out occurred near harvesting time. On the other hand it would be impossible to gather in the harvest if the fall-out came down at that time. Exposure to large doses of radiation will kill plants. The lethal doses are much higher than for animals: for food crops, even for the most sensitive plants, the LD-50 is about 10 Gy and it goes up to about 200 Gy. At smaller doses the effects of exposure are reduced yield and height; both are dose-rate dependent, the effect being smaller the lower the rate at which the dose was delivered. Yield is more severely affected in the early reproductive state. Plants with a growing season limited by climatic conditions may produce no yield at all, even if they survive. Flowering and ripening of fruit is delayed by exposure to radiation. Exposure of seeds produces mutations, most of which are deleterious. Among trees, conifers are very sensitive to radiation, whereas deciduous trees are less sensitive. The LD-50 values for exposed trees range from 20 to 100 Gy. Grasses are more radiation-resistant and a dose of 200 Gy is needed to destroy grassland.