In the compact thalli a secondary development often takes place by the growth of rhizoid-like internal filaments.
These structures cannot 3, then be produced from the product of a single spore nor even from the thalli derived from any two spores.
The two kinds of 4, thalli Blakeslee considers to have a differentiation 5, of the nature of sex and he distinguishes them as (+) and (-) forms; the former being usually distinguished by a somewhat greater luxuriance of growth.
Moller also in 1887 succeeded in growing small lichen-thalli without their algal constituent (gonidia) on nutritive solutions; in the case of Calicium pycnidia were actually produced under these conditions.
The cylindrical branches of the fruticose forms are usually radially symmetrical, but the flattened branches of these forms and also the thalli of the foliaceous form show a difference in the cortex of the upper and lower side.