Culture studies and field observations on Peltigera praetextata (Flk.) Vain, provide evidence of the existence of a delicate state of balance between the co-symbionts of this lichen.
Excessive supply of nutrients to discs of the lichen results in the outgrowth of the phycobiont and consequent destruction of the symbiotic relationship.
The conjoint effect of excessive moisture and light is shown to bring about similar symbiotic breakdown.
It is suggested that these three factors acting together as a growth-regulating system are chiefly responsible for the success of the lichen symbiosis.