The spermatophytes or phanerogams, also known as "seed plants", are a large monophyletic lineage of plants that belongs to the group of lignofitas (woody plants) and in which both angiosperms (flowering plants) and gymnosperms (conifers and Similar).
Spermatophytes form a separate group from lignophytes thanks to the shared characteristic of seed development, which is described in textbooks as an "evolutionary novelty" for the group..
The word "spermatophyte" literally means "plants with seeds", as it comes from the Greek words "sperma ", which means seed, and "Fiton", what does plant mean.
Spermatophytes are one of the most important organisms on earth, as both angiosperms and gymnosperms are two extremely abundant and essential groups for the functioning of practically all terrestrial ecosystems..
If you think quickly, plants with seeds are probably the most familiar group to most people, not only from a nutritional perspective (since oils, starches and proteins are obtained from the seeds of many plants), but also from a landscape point of view.
The giant redwoods of California, the large and leafy trees of the Amazon rainforest, lilies and roses, rice, oats, corn, wheat and barley, among thousands of others, are spermatophytes..
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- The main characteristic of spermatophytes or phanerogams is the production of seeds after pollination, that is, as a product resulting from the fusion of two sex cells.
- They are photosynthetic organisms, that is, they have chloroplasts that contain chlorophyll, so they can convert light energy from the sun's rays into usable chemical energy.
- The body of these vegetables is divided into root, stem and leaves.
- Some spermatophytes, the angiosperms, produce flowers and from these flowers originate the fruits, which are those that contain the seeds.
- Gymnosperms do not produce flowers, but have specialized structures to support the seeds..
- Most spermatophytes have well-developed vascular tissue, composed of xylem tissue and tracheids..
- They are widely distributed over the biosphere, thus occupying hundreds of different habitats.
- They may or may not have secondary growth tissues.
Flowering plants (angiosperms) grow in virtually any habitable region on earth (except coniferous forests), and can even dominate some aquatic ecosystems. Therefore, they are able to inhabit:
- Deserts
- Plains
- Mountainous areas
- Oceans, seas and rivers
Similarly, gymnosperms, other plants with seeds, also have great plasticity with respect to the habitat they can occupy, although they are more restricted to terrestrial and non-aquatic environments..
Seed plants belong to the Spermatophyta division. In this division are grouped the ferns with seeds "Pteridosperms", the Gymnosperms and the Angiosperms.
Seed ferns are a group composed mainly of fossil plants, so spermatophytes are often considered to be Gymnosperms and Angiosperms..
The word "gymnosperm" means "plants with bare seeds" (gymnos, which means "naked" and sperm, which means "seed").
Depending on the study being analyzed, this group of plants is a "non-natural" group, since its members are of paraphyletic origin, which means that not all of them have the same common ancestor; or it is a monophyletic group, brother of the angiosperms.
- The members of the group meet in this division because they share the common characteristic (apomorphy) of not producing flowers.
- In addition, these plants have structures known as "cones", some female and the other male..
- Seeds are not encapsulated within the wall of a fruit after fertilization.
- They have rolled, needle-shaped leaves that are abundant in waxes..
Gymnosperms are divided into the following lineages:
- Cycadophyta, the lineage thought to be the most basal
- Ginkgophyta
- Coniferophyta, conifers
- Gnetophyta o Gnetales, sometimes classified within the group of conifers
Flowering plants are, on the contrary, a proven monophyletic group, considered a sister group to gymnosperms. They are by far the most abundant, diverse and successful group of plants of all, comprising more than 95% of all plant species alive today..
They are also one of the most important plants for the world economic system, since they are not only exploited for the production of food, but also for the extraction of various types of raw material..
- All angiosperms have flowers, usually bisexual (both sexes on the same flower).
- Its seeds are encapsulated in an ovary, which develops into a fruit.
- They generally exhibit double fertilization.
Angiosperms comprise an extremely abundant and diverse group, whose classification is the object of study of many specialists in the field, so there are some discrepancies between one classification and another. However, among the most accepted is that this group includes the clades:
- Amborellales
or Nymphaeales
- Austrobaileyales
- Magnolides
or Laurales
or Magnoliales
or Canellales
o Piperales
or Monocots
or Commelinidos
or Eudicotyledons
The life cycle of spermatophytes is known as "sporic", where the sporophyte predominates and seeds are produced and the gametophyte, unlike other groups of plants, is reduced inside the ovule or the pollen grain.
From this it is understood that all plants with seeds have alternation of generations, one gametophytic and another sporophytic, but the gametophyte only develops when the plants reach adulthood or reproductive stage..
The sporophytes are those that carry the specialized structures where the female and male gametophytes are produced. Microsporangia produce pollen grains (male) and megasporangia produce megaspores or ovules (female).
In some cases, both the megasporangium and the microsporangium are found in different individuals or structures (Gymnosperms) but, generally, in most plants, both are in the same structure known as a flower (Angiosperms).
A flower is a specialized structure for reproduction and arises from the stem as an "extension" of the body of the plant..
The megasporangium contained in the flowers has a "container" (the ovary) that functions in the reception of the pollen grains, which are produced by the microsporangium (of the same flower or of different flowers).
The ovules within the ovary have all the necessary nutrients to support the development of the embryo, the seed and the fruit, a process that occurs after pollination and fertilization of the ovule by a pollen grain.
The seeds thus produced can be dispersed by different means and, once they germinate, form a new sporophyte that can repeat the life cycle..
Spermatophytes are extremely diverse plants, with very different life cycles, shapes, sizes, and ways of life.
To this group belong all the flowering plants that we know, practically all the plants that we consume as food and the large and majestic trees that make up the forests and jungles that support the life of animals..
- The apple, typical of the autumn season in many seasonal countries, belongs to the species Malus domestica, It is part of the Magnoliophyta division and the Rosales order.
- Pinus mugo It is a species of shrub pine that grows in the Alps and from which some compounds with expectorant, antiasthmatic and disinfectant properties are extracted..
- The bread that man consumes daily is made with flours produced from the seeds of wheat, a species of angiosperm belonging to the genus Triticum and that receives the name of Triticum aestivum.
The evolution of seed plants is closely related to the evolution of two structures: seeds and pollen grains..
The evolution of seeds is a process that occurred in several steps, but the exact sequence of them is not known, and it may have happened that two or more occurred at the same time. Next, the "steps" of the evolution of seeds are presented as some authors propose:
The term refers to the formation of two types of haploid spores (with half the chromosomal load of the plant that gave rise to them) inside two different sporangia.
- Megaspores: large and small in number, produced by meiosis in a structure known as megasporangium. Each megaspore develops inside the female gametophyte, in which archegonia are found.
- Microspores: the meiotic products of the microsporangium. Microspores originate from the male gametophyte, in which the antheridia are found.
It is considered as one of the essential "steps" during the evolution of spermatophytes, because the ancestral condition consisted of homosporium, that is, the production of a single type of spores (equal spores)..
In addition to the formation of two different types of spores, the spermatophytes developed another condition known as endosporia, which consists of the complete development of the female gametophyte inside the original wall of the spore..
The ancestral condition is known as "exosporia" and has to do with the germination of the spore and its growth as an external gametophyte..
Plants with seeds are characterized by producing a single megaspore, a characteristic that is thought to have evolved in two ways.
Initially, they had to have acquired the ability to reduce the number of meiosis cells inside the megasporangium to just one; It is important to note that each of these cells is known as a megasporocyte or megaspore stem cell..
After meiosis, a single diploid megasporocyte gives rise to 4 haploid megaspores. Three of these megaspores "abort", leaving a single functional megaspore, which increases in size, which is correlated with the increase in size and nutritional resources in the megasporangium..
One of the ancestral conditions or characteristics of spermatophytes is that the megaspore is released from the megasporangium, something that changed in this group, since in these plants the megaspore, once produced, is retained within the megasporangium..
This new evolutionary "acquisition" was accompanied, in turn, by a reduction in the thickness of the megaspore cell wall..
Many authors consider this to be one of the last events that took place during the evolution of seed plants. It is the "covering" of the megasporangium by a special tissue called the integument, which almost completely surrounds it, with the expression of the distal end.
The integument grows from the base of the megasporangium, which can be referred to in many texts as nucela.
Fossil records show that the integument first evolved as two separate lobes, however all seed plants that exist today have an integument consisting of a continuous covering surrounding the nucela except for the micropyle, which is the extreme distal.
The micropyle is the entry site for pollen grains or the pollen tube during the fertilization of the megaspore, so it actively participates in this process.
The evolution of the seeds was directly accompanied by the evolution of pollen grains, but what is a pollen grain??
A pollen grain is an immature male endosporic gametophyte. The endosporium in these structures evolved in a similar way to what happened in the seeds, since it involved the development of the male gametophyte inside the walls of the spore..
They are immature because when they are released they are not yet fully differentiated.
Unlike other types of plants and as discussed above, pollen grains are very different from megaspores. These are extremely small male gametophytes, which consist of a few cells.
When released from the microsporangium, the pollen grains must be transported to the micropyle of the ovule in order for fertilization to take place. The ancestral character of the pollination was anemophilic (pollination by wind).
Once in contact with the ovum, the male gametophyte completes its development by dividing by mitosis and differentiating. From this an exosporic pollen tube grows (outside the spore), which functions as an organ for the absorption of nutrients around the sporophytic tissue..
All seed plants that exist today have male gametophytes capable of forming a pollen tube shortly after coming into contact with the tissue of the megaspore (the nucela). The formation of the pollen tube is known as syphonogamy.
In addition to functioning as an organ for the absorption of food, the pollen tube functions in the delivery of sperm cells to the "egg" of the ovum..
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