6.1 The seed #
6.1.1 Three generations in one unit #
A seed contains an embryo (the next sporophyte, diploid), a food reserve, and a seed coat developed from the integument of the ovule (tissue of the parent sporophyte, diploid). In a gymnosperm the food reserve is the female gametophyte itself, haploid. A pine seed is therefore three generations: coat 2n, food n, embryo 2n. Calling that food “endosperm” copies a word that belongs to the triploid product of double fertilisation (Chapter 7). The tissues are not the same.
An ovule is a megasporangium (the nucellus) plus one or more integuments, with a micropyle as the opening. One megaspore develops inside it. After fertilisation and embryo development the ovule is a seed. An unfertilised ovule is not yet one. Pollen is a microspore that has started the male gametophyte, usually before or during dispersal. The microsporangium of a seed plant is a pollen sac.
6.1.2 What a spore cannot do #
A spore is one meiotic cell. It must build a gametophyte before any new sporophyte exists. A seed is dispersed only after an embryo exists, with food and a coat. The female gametophyte never becomes a free prothallus, and the pollen grain and its tube do not release sperm into environmental water. Cycad and Ginkgo sperm are still flagellated, but they swim only inside the ovule (6.3).
The parent spends heavily on each unit. What it gains is dormancy, a provisioned seedling, and dispersal of the next sporophyte rather than of a cell that must still find a mate.
6.2 The pine life cycle in full #
The study of The pine life cycle in full provides fundamental principles governing cellular architecture, molecular interactions, and physiological regulation.
Moss Life Cycle
6.2.1 Two cones #
Pinus is monoecious. Microsporangiate cones are small, short-lived, and usually lower on the shoot. Each microsporophyll bears two microsporangia on its underside. Microspore mother cells undergo meiosis. Each microspore becomes a four-celled pollen grain: two prothallial cells, a generative cell, and a tube cell, with two air bladders (sacci) from the exine. That grain is the immature male gametophyte, haploid. The bladders aid wind travel and orientation on the pollination drop.
Megasporangiate cones are larger, usually higher, and persist for years. The woody scale of a mature cone is an ovuliferous scale in the axil of a bract. The cone is a compound structure, not a simple stack of sporophylls. Each scale bears two ovules on its upper surface, micropyles towards the axis. One megaspore mother cell undergoes meiosis. Of the linear tetrad, three megaspores degenerate. The functional megaspore, the one farthest from the micropyle, grows by free nuclear division and then wall formation into the female gametophyte, which bears a few archegonia, often two or three, each with one large egg. This takes months. The female gametophyte is also the future food tissue.
6.2.2 The drop, the delay, and the embryo #
Pollen is wind-borne and lands in a pollination drop at the micropyle. Withdrawal of the drop carries the grains to the nucellus. The generative cell divides into a stalk cell and a body cell, and the body cell later makes two non-motile sperm. The tube grows slowly and pauses while the female gametophyte and its eggs are still forming. In many pines, fertilisation is about a year after pollination. One sperm fuses with the egg. The other degenerates. There is no second fusion that builds a nutritive tissue, as there is in a flowering plant. More than one embryo may start, from several archegonia or by cleavage of one embryo, and one usually fills the seed.
A suspensor pushes the embryo into the female gametophyte, which is the food tissue and is rich in fat and protein in many pines. The embryo has a radicle, a shoot apex, and several cotyledons, commonly about eight. The integument becomes the seed coat, often winged with tissue from the scale. The scales open as they dry. The female gametophyte has never been a free green prothallus.
6.3 Living gymnosperms #
Current molecular evidence generally makes the living gymnosperms a clade, sister to the angiosperms. Cycads and Ginkgo branch early; conifers are the large remainder; gnetophytes sit with the other gymnosperms, as sister to the conifers or nested among them. That internal seat is not settled. The anthophyte claim, that gnetophytes are the sister group of the flowering plants, rested on vessels, reduced strobili, and a second fertilisation. A second fusion does occur in some gnetophytes. It yields another embryo or a transient tissue, not persistent triploid endosperm, and molecular data have not supported the sister-group claim. The molecular position has been difficult. Gnetophytes are placed here within or beside the other gymnosperms, and the shared-looking traits are treated as convergence.
| Group | Distinctive points | Living diversity |
|---|---|---|
| Cycads | Dioecious; stout stems, pinnate leaves; coralloid roots with cyanobacteria. The pollen tube is a haustorium. Huge flagellated sperm swim the last distance inside the ovule. Often insects, not only wind | A few hundred species |
| Ginkgo | One species. Fan leaves, long and short shoots. Paired ovules, no cone. Flagellated sperm (Hirase, 1896). Fleshy, foul seed coat; not a fruit, because there is no ovary | One species |
| Conifers | Woody, usually needles or scales, resin ducts. Mostly monoecious and wind-pollinated. Taxus has an aril; juniper scales become fleshy. Non-motile sperm, delivered by the tube (siphonogamy) | About 600 species |
| Gnetophytes | Ephedra, Gnetum, and Welwitschia only. Vessels in the xylem. Welwitschia grows two leaves from a basal meristem for life | Three genera |
Fossil seed plants extend the grade beyond these four living clades. Wind is not the rule for the whole grade: many cycads are insect-pollinated, and in cycads and Ginkgo the tube is mainly a haustorium.
6.4 Key terms, summary, and self-check #
Key terms. Ovule; integument; micropyle; nucellus; pollen; pollination drop; ovuliferous scale; female gametophyte; siphonogamy; anthophyte hypothesis.
Summary. A gymnosperm seed packages a diploid embryo, a haploid female gametophyte as food, and a parental seed coat. It is not a spore and its food is not endosperm. In pine, wind carries a four-celled winged pollen grain to a pollination drop on an ovuliferous scale; the tube waits about a year while the female gametophyte builds archegonia, and non-motile sperm then fertilise an egg. Cycads and Ginkgo still have flagellated sperm inside the ovule. Conifers are the large living group. Gnetophytes, whatever their exact seat among gymnosperms, are not treated as the sister group of the angiosperms.
Self-check.
1. Name the three generations in a pine seed and the ploidy of each. Why is the food not endosperm?
2. What is a pollination drop for, and why is fertilisation not immediate?
3. What structure is the woody scale of a pine cone, and where are the ovules?
4. How does sperm delivery in Ginkgo differ from sperm delivery in pine?
5. Why do these notes reject the claim that gnetophytes are sister to the flowering plants?
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