THE SEED HABIT

The seed (text with tooltip) Unit of sexual reproduction in some plants. Formed when an ovule is fertilized and comprised of outer coat that encloses stored food and an embryo. and the associated pollen (text with tooltip) The collective mass of grains produced within the anthers of flowering plants or the male cones of a gymnosperm. In all seed plants, pollen is generated by the development of a microspore into a microgametophyte. The germination of the pollen grain leads to the development of a pollen tube, which delivers two sperm or sperm nuclei to the egg in the ovule. In flowering plants, mature microgametophyte has only two cells, a tube cell and a generative cell. organs are among the most elegant structures ever produced by evolution. The seed is formed by a heterosporic plant that retains the megasporangium in a spore wall (also called a nucellus (text with tooltip) Central part of a plant ovule; contains the embryo sac. ) which is surrounded by integuments (derived leaves). After the megaspore germinates to form an enclosed megagametophyte, the structure is called an ovule (text with tooltip) An ovule is a structure that contains the megagametophyte in seed plants. The megagametophyte remains within the megasporangium (the nucellus), which is surrounded by layers of integuments. After fertilization, the ovule develops into a seed. . The integuments do not completely enclose the ovule, but form an opening (a micropyle (text with tooltip) An opening in the integuments of an ovule that exposes part of the megasporangial wall (a chamber called a pollen chamber in gymnosperms). Thus, in gymnosperms, pollen enters the micropyle and germinates in the pollen chamber. However, because the micropyle is not exposed in flowering plants, their pollen germinates on the stigma. The pollen tube grows through the style, and enters the ovule through the micropyle. ) through which pollen can access an internal pollen chamber (text with tooltip) In gymnosperms, a cavity just above the nucellus in the ovule, the site where pollen accumulates and germinates. . Pollen, a mature microgametophyte within its microspore wall, germinates in the pollen chamber and forms a pollen tube which grows into the nucellus as a feeding haustorium. After the pollen tube enters the archaegonial chamber (text with tooltip) The archaegonial chamber is a small cavity between the nucellus and megagametophyte into which the archegonium (or its reduced derivative) exposes the egg to the pollen tube. , the microgametophyte delivers two sperm to the megagametophyte. In cycads and ginkgos the sperm is flagellated, but it is amoeboid in conifers, gnetophytes, and flowering plants. The mechanism for sperm nucleus delivery is further derived in flowering plants and gnetophytes; consult those descriptions for details. In the gymnosperms, the mature megagametophyte functions as a food storage tissue (called endosperm (text with tooltip) The nutritive storage tissue that grows from the fusion of a sperm cell with polar nuclei in the embryo sac. ) for the developing embryo and germinating seed. See Figure 1 for a general diagram of the seed habit. Thus, the structure that contains the outer integuments (now a seed coat), the nucellus (both derived from the parental sporophyte), the megagametophyte, and the embryonic plant (daughter sporophyte) contains three generations and is called the seed.

Because the ovule cannot move, the pollen must be able to transfer to the pollen chamber. Thus, the development of pollen is similar to that of eusporangiate ferns. Microspores form within eusporangiate staminate structures, which usually are clustered into pollen-bearing organs (see Figures 2 C and 4). Microspores develop into pollen by internal, usually unequal divisions with initial cells, germinative cells (that will form the sperm) and the tube cell (that forms the pollen tube). Thus, a mature pollen microgametophyte has very few cells. The microspore wall is made of several walls (intine and exine). In many pollen grains, the layers separate and the exine forms a ballooning structure which facilitates wind pollination.

According to Linkies et al. (2010), the seed represents several evolutionary innovations that are notable:

• the peculiar anatomy and development of ovules, pollen, and seeds
• the development of endosperm as nutritive tissue for the developing embryo and emerging plant
• the ability of the seed to enter into an extended period of dormancy

Some of the earliest seeds (see Figures 2 A & B, and 3 for reconstructions) were on the undersides of of certain modified leaves. The megasporangium was stalked and surrounded by an integument that came together to form a beak with a small opening (the micropyle) into which pollen could be caught and germinate to produce the pollen tube that delivers the sperm to the egg. The earliest ovules had only a single integument surrounded by a cupule produced by an outer series of integumentary segments, which become a second layer of integuments in more advanced ovules. The pollen-bearing organs were eusporangiate microsporangia packed together underneath a modified umbrella-like leaf segment (see Figures 2 C & D and 4 for reconstructions).

The basal clades of the seed ferns, taxa which occurred during the Paleozoic, are nested (see Figure 5), and, therefore, paraphyletic. Other taxa assigned to the seed ferns are scattered in the seed plant tree. The peltaspermopsids persisted from the Permian to the Triassic and are sisters to the Ginkgo + Conifer + Gnetophyte clade. The caytoniopsid clade is also known as the higher seed ferns (Hilton and Bateman (2006). These form a sister relationship with the cycadedoids (also called bennetaleans), which together are sisters to the flowering plants. Figure 6 suggests the polyphyletic nature of the pteridosperms and that they may represent at least 5 different extinct higher taxa. Furthermore, the cycads likely represent a surviving line of the Paleozoic pteridosperms.

THE PALEOZOIC SEED FERNS: LYGINOPTERIDOPSIDA, MEDULLOSOPSIDA, CALLISTOPHYTOPSIDA

In general much is known about the anatomy and developmental life history of these plants through detailed preservation in coal and coal balls.

The Lyginopterid seed ferns were common plants in the Carboniferous period. As their name implies, these plants had a fern-like appearance. However, they bore microsporangia (producing pollen; see Figure 2 C & D) and ovules (Figure 2 A & B) on the undersides of certain modified leaves. They were small plants with stems no larger than 3cm in diameter. The lyginopterids formed part of the understory of the great coal age forests dominated by Lepidodendron and Calamites. The leaves were fern-like and up to 0.5m long. Seeds were borne on the margins of the fertile leaves.

The Medulosid seed ferns of the Carboniferous period were much more robust than the Lyginopterids. The plants looked like palms with large leafless trunks (up to 5m high) and sprays of large leaves at the top (Figure 6). Secondary growth was accomplished by a polycyclic stele. The leaves had sunken stomata much like those of the Cycadophyta. Their ovules had at least two integuments.

The Callistophytan seed ferns (Figure 7) made up the third major group of Paleozoic pteridosperms (Carboniferous to Permian). Their stems were relatively small (no thicker than about 2-3 cm in diameter). They seem to have been vines growing much as wild grapes in the Eastern Deciduous Forest. Although the stems were small, they were eustelic with well-developed secondary growth. The seed, though small, had three integumentary layers, and the pollen was saccate (text with tooltip) In the form of a sac. .

THE MESOZOIC SEED FERNS: PELTASPERMOPSIDA AND CAYTONIOPSIDA

Like most of multicellular life, the seed ferns experienced a bottleneck at the Permian-Triassic boundary. Then a mass extinction, the time referred to as the Great Dying, occurred in which between 90-95% of all genera disappeared. Many larger groups like the trilobites that had been decimated by earlier extinctions disappeared completely. In many cases, those few taxa that survived formed the basis for a new radiation. That was the case of the seed ferns. The Mesozoic seed ferns were very different from the basal clades with sophisticated cupules that contained ovules. In fact, the ovule-bearing organs of Mesozoic seed ferns almost completely enclosed the ovules in structures that looked remarkably like the ovularies of flowers (Taylor et al. 2006).

The peltaspermalean seed ferns appeared in the lower Permian and persisted through the mass extinction into the Triassic. The corystospermaleans were quite successful during the Triassic period. They tended to be small, woody plants with ovules that were recurved (Figure 8). Peltaspermales also were small shrubs or vines with fern-like leaves with ovules associated with disc-like structures on arranged as open strobili on an axis (Figure 9).

Most of the Mesozoic seed ferns were the caytoniopsids (Figures 10-11). Most taxa in this class did not have typical fern-like leaves, but tended to have compound strap-like leaves, Members of the Caytoniales had the complex recurved ovule-bearing organs and saccate pollen produced by complex pollen-bearing organs and were the most common members of the seed ferns in the Jurassic and Cretaceous periods. The glossopterids and gigantopterids occurred through the Permian and into the Triassic Periods. The gigantopterids are known by many leaf fossils and some seeds, but thier overall growth habit is unknown. Indeed, they may be a form group, and, therefore polyphyletic. The glossopterids had monopodial growth and a growth habit that resembled conifers (Figure 11). They had a cluster of seeds attached to a common receptacle (text with tooltip) The end of the stem which bears the flower parts. in the axils of certain leaves. Because the glossopterids survived the Great Dying at the end of the Permian and grew all over Pangaea, particularly Gondwanaland, their fossil remains were used as evidence by Alfred Wegener to show that the southern continents, particularly Africa and South America, had once been joined.