The development of many complex stratified lichen thalli is made through stages of complex phenotypic interactions between a filamentous fungus (the mycobiont), and a trebouxioid alga (the photobiont). Typically, the second stage of this symbiotic development is marked by the envelopment of the photobiont by the mycobiont through increased lateral hyphal branching and the formation of appressoria. Previously, the mycobiont’s envelopment of photobiont cells was considered thigmotropic (a growth response due to shape) as a mycobiont can envelop algal sized objects in its environment. However, after growing the mycobiont Cladonia grayi with various phototrophs and glass beads, we conclude that the mycobiont does not show this characteristic second stage morphological response when grown in non-compatible pairings. Instead, C. grayi displays a distinctive morphological growth response only in compatible symbiotic pairings, such as with its natural photobiont Asterochlor’is sp.