ABSTRACT
The specific developmental patterns throughout the ontogeny of plants depend on regulation by environmental factors. The most important external factor is light. Two fundamentally different aspects of light action on green plants have to be distinguished. The processing of light signals in plants starts with the perception of the stimulus by the photoreceptor, resulting in initiation of primary biochemical interactions at the cellular level. The cascade of the proceeding physiological reactions finally results in the constitution of the morphogenetic response. The first and probably the most fundamental step in protonemal development is the constitution of polarity in the course of spore germination. The formation of a main axis becomes obvious when a filamentous germ tube protrudes from the spore. Spore germination up to this stage has been described to be affected by light. The filamentous protonema originates from the elongating germ tube providing the structural pattern for the subsequent formation of gametophores.
