Plant Kingdom - Pteridophytes

Pteridophytes are the first terrestrial plants to possess vascular tissues, specifically $Xylem$ and $Phloem$, used for the conduction of water and minerals.

The main plant body is a sporophyte ($2n$), which is differentiated into true roots, stem, and leaves, unlike bryophytes where the dominant phase is the gametophyte.

Leaves in pteridophytes are either small (microphylls), as seen in $Selaginella$, or large (macrophylls/megaphylls), as seen in most ferns.

Sporangia are produced on leaf-like appendages called sporophylls. In some genera like $Selaginella$ and $Equisetum$, sporophylls form compact structures called strobili or cones.

Spores germinate to produce a small, multicellular, free-living, mostly photosynthetic thalloid gametophyte called the prothallus ($n$).

Water ($H_2O$) is essential for fertilization, as it is required for the transfer of flagellated male gametes (antherozoids) to the archegonium.

Most pteridophytes are homosporous (produce one type of spore), but some like $Selaginella$ and $Salvinia$ are heterosporous, producing megaspores and microspores.

Heterospory is considered a significant evolutionary step toward the seed habit because the female gametophyte is retained on the parent sporophyte for a period.

The classification includes four classes: Psilopsida (e.g., $Psilotum$), Lycopsida (e.g., $Selaginella$), Sphenopsida (e.g., $Equisetum$), and Pteropsida (e.g., $Pteris$, $Adiantum$).