Question

What is the difference between primary and secondary protonema?

Answer 1

Hint: Protonema is basically defined as the creeping, green, branched, and often filamentous stage of the process of growth. Protonema is a haploid, autonomous and gametophytic stage of the moss cycle. A protonema is a thread-like connection of the cells that form the gametophyte's beginning stage of growth in the life cycle of the mosses.

Complete answer:
When the growth of a moss takes place from a spore, then it starts as a germ tube that lengthens and branches into a filamentous complex known as a protonema, which further develops into a leafy gametophore, the adult form of a gametophyte in bryophytes.
The process of development is as follows: 1. Firstly, the moss spores germinate to form a protonema, which is an alga-like filamentous structure. 2. It is a representation of a young gametophyte. While the growth of a protonema takes place by the apical cell division process, under the influence of the phytohormone cytokinin, buds that develop by three-faced apical cells are induced at some stage. 3. The gametophores, roots, and leaf-like structures are formed due to this process. The bryophytes lack true leaves (megaphyll). The formalized paraphrase protonemata are generally found in all the available mosses and some of the liverworts. Its development takes place from a spore and it finally gives rise to new plants. Funaria, Polystichum, and sphagnum are some examples.

Primary protonema	Secondary protonema
The development of primary protonema takes place from the spores	Secondary protonema is developed from structures like rhizoids and stems.
Primary protonema is responsible for only the germination of spores	Whereas, secondary protonema is responsible for the overall development of the gametophytic part of the plant.
Primary protonema is the basic part of this chain which is formed from a spore.	Secondary protonema develops from any of the parts of the adult which may include stems, leaves and rhizoids.

Note: Moss protonemata are basically made up of two different types of cells: chloronemata, which develops upon germination, and caulonema, which gets separated from the chloronemata and form buds, which then differentiate to gametophores.