Trees are important to the environment owing to their ecological services. However, many aspects of their form and function are poorly understood by the public. From their earliest years, children have an elementary knowledge about plants which they gain from their everyday observations, their parents and other people and from their kindergarten and primary schooling. The goal of this research was to investigate Polish children's understanding of trees' internal structure. This cross-age study involved 5-year-old children from kindergarten (n = 57, 26 boys and 31 girls) and 7-year-old children (n = 105, 57 boys and 48 girls) and 10-year-old children (83 children, 36 boys and 47 girls) from primary school. Participants were asked to draw the internal structure of a tree. The results of the study showed that there were some significant differences in the responses between age groups and between the genders. Nevertheless, there were some ideas that were shared among all age groups indicating that they might be resistant to change. The study also identified some alternative conceptions about the internal structure of plants and the influence of the media on children's ideas.