EVOLUTION OF THE SEED
Evolution of the seed
Seeds (known as ovules before fertilization) derive from the megasporangia of heterosporous plants that develop only one spore and are retained on the parent plant. The gametophytes are reduced and enclosed within the spore wall. They have a protective integument, perhaps formed from sterile megasporangia. Microspores are known as pollen grains.
These were trees of Devonian and Carboniferous times and had a combination of wood resembling that of conifers, leaves that may have been microphylls and sporangia resembling those of eusporangiate ferns. They may be ancestral to seed plants and/or ferns.
Seed plants first appeared in the Devonian period and may be descended from ferns or Trimerophytopsida. Some Devonian fossils, known as seedferns, had fern-like leaves and were heterosporous, showing intermediate stages in integument development
The seed habit derives from a heterosporous condition similar to what is seen in some living lycopsids and ferns (Topics Q2 and Q3). In these there are two types of spore, microspores that give rise to the microgametophyte bearing sperms but no eggs, and megaspores that give rise to themegagametophyte bearing an egg or eggs. The gametophyte of both sexes is much reduced in size in all heterosporous plants, and, in most, it grows entirely within the spore wall. In many, the whole megasporangium is retained on the parent plant until the megagametophyte is fertilized. The seed derives from such a heterosporous condition, the seed itself being the megasporangium. It is a megasporangium that always matures just one spore, the three others produced by meiosis aborting and is known as the ovule before fertilization. It does not dehisce and the main body of the sporangium becomes a mass of parenchyma cells known as the nucellus . The microspores are usually (but not always) smaller than the megaspores and the microgametophyte is limited to a few cells inside the spore wall. The microspores of seed plants are the pollen grains, the microsporangium theanther .
In the seed the nucellus (megasporangium) is surrounded, except for a small opening, by another structure, the integument and it is this structure that distinguishes a seed from other heterosporous plants . The origin of the integument is obscure, though the most likely possibility is that it is derived from the fusion of a number of sterile sporangia surrounding the fertile one.
There are some intermediate stages represented among early fossil seed plants. The megagametophyte is reduced in size and retained entirely within the sporewall (itself retained in the sporangium), and it ruptures by the gap in the integument. All seed plants except the flowering plants and some Gnetales produce recognizable archegonia by the rupture. In the flowering plants the megagametophyte is reduced to the embryo sac , normally eight nuclei, within the spore wall, the synergid cells perhaps representing what remains of the archegonium. In all seed plants the seed is retained on the parent sporophyte until after a new embryo starts to grow and only then is it dispersed as a whole unit. It will be seen that this seed consists of parts of three different genetic constitutions: an outer integument and nucellus (megasporangium) from the parent sporophyte, the haploid female gametophyte inside this and the new developing sporophyte inside this.
In all the gymnosperms (literally ‘naked seed’) the seed is exposed, usually on a modified leaf, but in the flowering plants, or angiosperms(literally ‘hidden seed’) it is enclosed by the ovary that becomes the fruit at maturity. It is suggested that this derives from a cup-like outgrowth from the sporophyte which is seen in some gymnosperms .
The Progymnospermopsida are regarded as the likely ancestral group of all seed plants. It is an entirely fossil group found early on in the evolution of land plants from mid Devonian to early Carboniferous rocks (Table 1). They were trees with well-developed trunks and lateral branches. The trunks had marked secondary thickening, unlike any ferns, and closely resembled trunks of living conifers (Topics C4 and R2). Their leaves were quite small but densely packed on the lateral branches and resembled microphylls . Some of the side branches terminated in sporangia, of the eusporangiate type , like contemporary fossil ferns, but some may have been heterosporous.
The striking feature of this group is its combination of sporangia typical of primitive ferns with wood typical of conifers and leaves that may be of the microphyll type. The progymnosperms are regarded by some as the ancestors of modern ferns as well as seed plants. Whatever the precise origins of the other seed plants, the progymnosperms clearly occupy an intermediate position and provide a fascinating link between early spore-bearing plants and seed plants.
Fossil seed plants first appear in Devonian rocks and they are abundantly represented as fossils from the mid-Devonian period onwards. The relationship between seed plants and ferns is not clear, with some researchers maintaining that all seed plants evolved from a fern-like ancestor, others suggesting an independent origin from the Trimerophytopsida or other primitive group for some or all seed plants. Seed plants are highly variable in structure and it is possible that the seed habit has evolved more than once.
Many fossils of vegetative parts that closely resemble ferns from the mid- Carboniferous period onwards were heterosporous, and some retained the megagametophyte on the leaves until after fertilization with the embryo
beginning to grow. These show intermediate stages in the evolution of the integument, and are regarded as the earliest seed plants, the ‘seed-ferns’ orpteridosperms .