SPERMATOPHYTES OR PHANEROGAMS: CHARACTERISTICS, CLASSIFICATION, EVOLUTION - BIOLOGY - 2025

The espermatofitas or flowering plants, also known as "seed plants" are a great monophyletic lineage of plants belonging to the group of lignofitas (woody plants) and which are classified both angiosperms (flowering plants) as gymnosperms (conifers and the like).

Spermatophytes form a separate group from lignophytes thanks to the shared feature of seed development, which is described in textbooks as an "evolutionary novelty" for the group.

Photograph of an apple tree, a spermatophyte plant (Source: W. carter / CC0, via Wikimedia Commons)

The word "spermatophyte" literally means "plants with seeds", as it comes from the Greek words "sperma", which means seed, and "phyton", which means plant.

Spermatophytes are one of the most important organisms on earth, as both angiosperms and gymnosperms are two extremely abundant groups essential for the functioning of practically all terrestrial ecosystems.

If you think about it 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.

Spermatophytes are 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.

Characteristics of spermatophytes

- 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 the 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, angiosperms, produce flowers and from these flowers originate the fruits, which are those that contain the seeds.

- Gymnosperms do not produce flowers, but they have specialized structures to support the seeds.

- Most spermatophytes have a well-developed vascular tissue, composed of xylem tissue and tracheids.

- They are widely distributed over the biosphere, so they occupy hundreds of different habitats.

- They may have tissues with secondary growth or not.

Habitat

Flowering plants (angiosperms) grow in virtually any habitable region on earth (except coniferous forests), and may even dominate some aquatic ecosystems. Therefore, they are able to inhabit:

- Deserts

- Plains

- Serranías

- 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.

Classification and taxonomy

Seed plants belong to the Spermatophyta division. In this division are grouped the ferns with seeds "Pteridosperms", Gymnosperms and Angiosperms.

Seed ferns are a group composed mainly of fossil plants, so spermatophytes are often considered to be Gymnosperms and Angiosperms.

Gymnosperms

Cones of a pine, a Gymnosperm (Source: Sridhar Rao / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons)

The word "gymnosperm" means "plants with naked 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 have the same common ancestor; or it is a monophyletic group, brother of 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 one male.

- The seeds are not encapsulated within the wall of a fruit after fertilization.

- They have rolled leaves, needle-shaped and abundant in waxes.

Gymnosperms are divided into the following lineages:

- Cycadophyta, the lineage thought to be the most basal

- Ginkgophyta

- Coniferophyta, conifers

- Gnetophyta or Gnetales, sometimes classified within the group of conifers

Angiosperms

Flowers of Tetradenia riparia, an angiosperm. Conrado / CC BY (https://creativecommons.org/licenses/by/3.0)

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, as they are not only exploited for food production, but also for the extraction of various types of raw material.

- All angiosperms have flowers, usually bisexual (both sexes in 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

o Monocotyledons

• Petrosavials
• Acorales
• Alismatales
• Asparagales
• Dioscoreales
• Liliales
• Pandanales

or Commelinidos

• Arecales
• Commelinales
• Zingiberales
• Poales

or Eudicotyledons

• Buxales
• Trochodendrales
• Ranunculales
• Proteals
• Berberidopsidales
• Dillenials
• Gunnerales
• Caryophyllales
• Santalales
• Saxifragales
• Rosides
• Vital
• Crossosomatales
• Geraniales
• Myrtales
• Zygophyllales
• Celastrales
• Cucurbitals
• Fabales
• Fagales
• Malpighiales
• Oxalidal
• Rosales
• Orchards
• Brassicales
• Malvales
• Sapindales
• Asterids
• Cornales
• Ericales
• Garryales
• Gentianales
• Lamiales
• Solanales
• Apiales
• Aquifoliales
• Asterales
• Dipsacales

Life cycle and reproduction

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.

Alternation of generations

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. The microsporangia produce the pollen grains (male) and the megasporangia produce the 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).

Flowers

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 pollen grains, which are produced by the microsporangium (from the same flower or from 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.

Examples of spermatophyte species

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 for food and the great 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 Malus domestica species, it is part of the Magnoliophyta division and the Rosales order.

- Pinus mugo 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 which is called Triticum aestivum.

Evolution of spermatophytes

The evolution of seed plants is closely related to the evolution of two structures: seeds and pollen grains.

- Evolution of seeds

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:

1-Heterosporia

The term refers to the formation of two types of haploid spores (with half the chromosomal load of the plant that gave them origin) inside two different sporangia

- Megaspores: large and small in number, produced by meiosis in a structure known as a 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 only one type of spores (equal spores).

2-Endosporia

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.

3-Reduction in the number of megaspores

Seed plants 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 within the megasporangium to just one; It is important to note that each of these cells is known as a megasporocyte or megaspore mother 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.

4-Retention of the megaspore

One of the conditions or ancestral 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.

5-Evolution of the integument

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 surrounds it almost completely, with the expression of the distal end.

The integument grows from the base of the megasporangium, which can be called nucela in many texts.

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.

- Evolution of pollen grains

The evolution of 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.

The pollen tube

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.

References

1. Merriam-Webster. (nd). Spermatophyte. In Merriam-Webster.com dictionary. Retrieved April 7, 2020, from merriam-webster.com
2. Nabors, MW (2004). Introduction to botany (No. 580 N117i). Pearson.
3. Simpson, MG (2019). Plant systematics. Academic press.
4. Raven, PH, Evert, RF, & Eichhorn, SE (2005). Biology of plants. Macmillan.
5. Westoby, M., & Rice, B. (1982). Evolution of the seed plants and inclusive fitness of plant tissues. Evolution, 36 (4), 713-724.