Growth and Form in Lower Plants and the Occurence of Meristems

Fungi and streptomycetes have a similar morphology and in both groups branching appears to be regulated in a similar manner. Both types of hyphae grow by tip extension but streptomycete hyphae never attain the extension rates commonly observed for fungi. Fungal hyphae are able to attain high rates of extension because a very large volume of protoplasm contributes to tip growth and because a vesicular growth system facilitates the rapid assembly of the tip wall. Growth of fungal and streptomycete mycelia involves the duplication of a physiological unit of growth which consists of a tip and a portion of hypha whose average length remains constant. However, it is not clear that growth of such mycelia is truly modular. Although hyphal fusions within a mycelium are common in higher fungi their significance in the organism's life style is not known. Growth in lower green plants, especially algae, is considered and the question of whether coenocytic algae are modular or not is discussed.