Sycamore (Acer pseudoplatanus L.) regenerates freely from seedlings in northern England. Even when temperatures are below average in early May, pollen germinates and pollen tubes grow to the base of the style within 3-5 d of the flowers opening. In (lowers at constant temperatures from 5.5 to 15.5°C, pollen germinates rapidly and a high proportion of pollen tubes reach the base of the style within four days. In natural conditions proembryos are present in fertile ovules 3 weeks after pollination. Fertile fruits grow rapidly until July and then more slowly until September. Large crops of fertile fruit are formed even when temperatures are below normal at flowering and subsequent development proceeds in both warm and cool summers.