Scanning electron microscope study of early lichen synthesis.

In the early stages of cellular interaction between the symbionts of Lecidea albocaerulescens, the phycobiont produced an extracellular sheath which bound to it hyphae of the mycobiont. Such a sheath may be a means by which the symbionts recognize each other. Hyphae of the mycobiont formed flattened appressoria as they grew over the algal cells and in this way secured the autotrophic population necessary for the development of a lichen thallus.