The Structure of Peltigera with Especial Reference to P. praetextata

Abstract

In the mature metathallus the following tissue regions are distinguished: upper cortex, gonidial layer, intergoni-dial hyphae, infragonidial hyphae, medullary hyphae,. hypothallus and rhizines. The thallus develops from meristem just within its actual margin, the proto-thallus arising from the medulla. The growth, development, and function of the rhizines is described in detail. Special attention is given to the structure and development of isidia. True isidia are limited to P. praetextata and P. lepidophora. In P. praetextata isidia arise (a) from cracks due to external agencies and (b) from special infragonideal hyphae that break through the cortex. Leaf-like isidia of P. praetextata show differentiations analogous to leaves of dicotyledonous land-plants and their function is probably photosynthetic. There is no indication that isidia are ever reproductive structures. "Peltigera is a typical land-plant and its ancestors must have developed as land-plants from the very earliest stages, probably from the very first lichen stage as such.".