In earlier paper 1, 2, 3 it has been shown that morphogenetically effective blue light — compared with red light — increases protein synthesis in young fern gametophytes (= sporelings). The induction of normal morphogenesis, i.e. formation of two-dimensional prothallia instead of filaments can only occur in the presence of blue light and is correlated with an increase of protein synthesis. — The present paper confirms the hypothesis that the blue light dependent increase of protein synthesis might be causally connected with blue light dependent morphogenesis. Under the influence of 5-methyl-tryptophane, an effective inhibitor of protein synthesis, morphogenesis of the sporelings is much more inhibited under blue than under red light. — The inhibition due to the antimetabolite can be reversed by the application of tryptophane. This reversion occurs under red light at a lower concentration of tryptophane and is more complete than under blue light.