The protonema is mainly composed of two types of cells. They are, chloronemata and caulonemata. The chloronemata develops during the early stages of germination which is then differentiated and developed into caulonemata.
What is a Prothallus?
Prothallus is the gametophyte stage appears in the life cycles of ferns and other pteridophytes. It is a heart-shaped structure which developed through the germination of a spore. This heart-shaped structure is a characteristic feature of pteridophytes’ life cycle. The prothallus has a short lifespan. Concerning its typical dimensions, the prothallus is 2 mm – 5 mm wide. It is composed of both male and female reproductive units namely antheridium and archegonium. Beneath the gametophytes, root-like structures which are known as rhizoids are extensively grown.
The typical structure of the prothallus could be varied according to the type of species. But the variations are minute and does not deviate largely from the normal typical prothallus structure. In some species of pteridophytes, prothallus contains chlorophylls which enable it to undergo photosynthesis. By photosynthesis, the nutritional requirement of the prothallus is fulfilled. The species which lacks chlorophyll and unable to photosynthesise, fulfill their nutritional requirement through the help of rhizoids and act as typical saprotrophs.