Predictive simulation models of lichen growth and distribution should be sensitive to the pattern of production and growth within thalli, and yet little work has dealt directly with the problem of intrathallus variability in production and thus growth.

It has been assumed for many years that lichens show marginal or apical growth as a result of the allocation of local supplies of photosynthate to the proliferating tissues. Only recently have intrathallus gradients in photosynthetic activity been demonstrated in foliose and fruticose species which mirror the observed pattern of growth. This present study aimed to determine whether there was intrathallus variation in production in the form of production gradients in Umbilicaria lichens.

Discs of tissue were removed from large thalli of four species of Umbilicaria and sampled for net CO2 exchange under constant conditions. The results were plotted as maps of physiological activity both on a weight and area basis, but showed an absence of clear intrathallus gradients in activity. This suggests that production is diffuse over the entire surface of the thallus and thus the growth pattern is diffuse.

These results are interpreted to mean that not all lichens grow apically and that greater research effort should be directed towards discovering the extent of variation in growth pattern amongst various families of lichens.