Why a Soaked Chana Seed Splits in Two but a Maize Grain Does Not

Bryophytes stay small and cling to damp places. Ferns grow taller thanks to pipes that carry water, pines scatter seeds without needing water for fertilisation, and flowering plants wrap their seeds inside fruits.

Each step is a new solution to life on dry land.

This part covers pteridophytes, homospory and heterospory, gymnosperms, and angiosperms with the split into monocots and dicots.

Pteridophytes are the first land plants with true vascular tissue — xylem and phloem — and their main plant body is a sporophyte with true roots, stem and leaves, while the gametophyte is a small, free-living prothallus.

Advances over bryophytes:

- Vascular tissue — xylem and phloem carry water and food
- Dominant sporophyte — the large, familiar plant is diploid
- True organs — roots, stems and leaves; leaves may be small microphylls (Selaginella) or large macrophylls (ferns)

Reproduction. Sporangia form on leaf-like sporophylls, sometimes grouped into cones called strobili, as in Selaginella and Equisetum. Spores formed by meiosis grow into a prothallus — a small, multicellular, usually photosynthetic, thalloid gametophyte that needs cool, damp, shady places.

Examples: Selaginella, Equisetum (horsetail), Pteris and Dryopteris (ferns), Adiantum (maidenhair fern), Salvinia.

Worked example — sorting the generations. In Pteris, the leafy fern you see is the sporophyte; the tiny heart-shaped green plate growing from a fallen spore is the gametophyte.

An everyday example. Ferns growing on moist roadside slopes in hill stations, and maidenhair ferns kept in shaded pots, are pteridophytes.

The substance. Pteridophytes still need water for fertilisation, which is why their gametophytes are restricted to damp places.

Homosporous plants make only one kind of spore, while heterosporous plants make small microspores that grow into male gametophytes and large megaspores that grow into female gametophytes; because the female gametophyte and even the young embryo are kept on the parent plant, heterospory is seen as a precursor to the seed habit.

Homosporous: most pteridophytes, including many ferns — all spores alike.

Heterosporous: Selaginella and Salvinia —

- Microspores — small, develop into male gametophytes
- Megaspores — large, develop into female gametophytes

The link to seeds. In heterosporous plants the female gametophyte stays on the parent sporophyte for some time. The zygote develops into a young embryo within the female gametophyte. This retention and protection of the developing embryo is exactly the event that, carried further, becomes a seed.

Worked example — spotting heterospory. A plant that produces two sizes of spores in separate sporangia, with the larger ones giving egg-bearing gametophytes, is heterosporous — like Selaginella.

An everyday example. A bird that keeps its egg safe in a nest rather than leaving it exposed gives its young a better start — just as retaining the female gametophyte protects the embryo.

The substance. Heterospory alone is not a seed — a true seed also needs a protective covering around the ovule, which appears in gymnosperms.

Gymnosperms are heterosporous seed plants whose ovules are not enclosed in an ovary, so their seeds lie exposed with no fruit wall; microspores and megaspores are produced in male and female cones.

Body features:

- Roots — usually tap roots; Pinus roots form mycorrhiza with fungi, and Cycas has coralloid roots housing nitrogen-fixing cyanobacteria
- Stems — unbranched in Cycas, branched in Pinus and Cedrus
- Leaves — large pinnate leaves in Cycas; needle-like leaves in conifers, with thick cuticle and sunken stomata to reduce water loss

Reproduction:

- Male cones — microsporophylls bear microsporangia that make pollen grains
- Female cones — megasporophylls bear ovules
- Pollen is carried by air to the ovules; a pollen tube delivers male gametes to the egg
- The fertilised ovule becomes a seed, which stays uncovered because there is no ovary

**Worked example — Cycas versus Pinus.

- Cycas: male and female cones on different plants
- Pinus: male and female cones on the same plant

An everyday example. Pine cones collected on walks in Shimla or Nainital** are the female cones of Pinus, with naked seeds sitting on their scales.

The substance. Gymnosperms do not need water for fertilisation — the pollen tube carries male gametes directly to the egg.

Angiosperms, the flowering plants, bear their ovules inside an ovary so their seeds are enclosed in fruits; they are divided into monocotyledons, with one cotyledon, parallel venation and floral parts in threes, and dicotyledons, with two cotyledons, reticulate venation and floral parts in fours or fives.

Distinguishing features of angiosperms: flowers carry the sex organs, seeds develop inside fruits, and they live in an enormous range of habitats — from tiny floating Wolffia to tall Eucalyptus trees.

Monocots versus dicots:

- Cotyledons: one versus two
- Leaf venation: parallel versus reticulate (net-like)
- Floral parts: in multiples of three versus four or five
- Roots: usually fibrous versus usually a tap root

Examples. Monocots: wheat, rice, maize, banana, onion, coconut. Dicots: mango, pea, gram, mustard, neem.

Worked example — identify the group. A plant with parallel-veined leaves and a flower with six petal-like parts is a monocot; a plant with net-veined leaves and five petals is a dicot.

An everyday example. A soaked chana (gram) seed splits easily into two halves — its two cotyledons — while a soaked maize grain stays in one piece.

The substance. Use several characters together — a few monocots, such as yams, have net-veined leaves, so one feature alone can mislead.