The stolons are modified stems characteristic of many plants with vegetative reproduction (asexual) projecting from the base of the main stem along the surface of the ground (are crawling) and develop adventitious roots, so that eventually can give place to a separate plant.
These structures also occur in animals and fungi and fulfill the same clonal or asexual propagation functions, forming genetically identical individuals connected to each other through non-segmented processes (stolons).
Photograph of a plant stolon (Source: Macleay Grass Man via Wikimedia Commons)
Living beings have the ability to increase the size of their natural populations by two reproductive routes: sexual and asexual. Some of them are exclusively sexual (humans and other mammals, for example) but others can reproduce both sexually and asexually (fungi, plants and others).
Sexual reproduction involves the fusion of a female gamete (the ovum) with a male gamete (the sperm or pollen grains), this fusion produces a zygote that will give rise to the embryo that will form a new individual genetically different from its two parents.
Sexual reproduction implies an increase in the genetic variability of the populations of living beings and, in many cases, represents a selective advantage, since the new individuals can better adapt to different environmental conditions, among other things.
On the other hand, clonal, asexual or vegetative reproduction has to do with the increase in the number of individuals in a population based on mitotic divisions of the same individual, thus being genetically identical individuals.
Stolons in plants
Stolons are characterized as projections from the stem that produce adventitious roots wherever they come into contact with the substrate (soil).
They arise from the "main" stem and as they are modified stems, they also divide into nodes, from which the adventitious roots (roots other than the main root) emerge. Furthermore, the portions of the internodes are of great length.
The form of growth by stolons consists, then, of a bud of the main stem that originates a stolon. In the first node that comes into contact with the ground roots are produced, and in the next the stolon apex acquires a vertical position and thickens to form a structure in which leaves and flowers are produced.
The stolon that "turned" upwards produces roots and new buds to project new stolons or, rather, to "continue" the stolon that had its origin in the initial plant. When the stolon dies, the “daughter” plants separate and are completely independent.
Since independent plants can be formed from stolons without the need for the fusion of two gametic cells to occur (ovum and pollen grain), these structures are one of the asexual reproduction pathways of certain plants that allows them to form " networks ”of clonal plants, which facilitates their dispersal, although it does not favor genetic variability.
Stolons of a strawberry plant (Fragaria ananassa) (Source: Frank Vincentz via Wikimedia Commons)
An example of plants with asexual reproduction by means of stolons are strawberries (Fragaria ananassa), whose mass cultivation exploits this capacity to obtain a large number of plants in considerably shorter times than those involved in the germination of sexual seeds.
Grasses also reproduce clonally through stolons, and wheat and grasses are good examples of these species. This type of reproduction is also true for some aromatic species of commercial interest such as mint or spearmint, etc.
Plant stolons are modified stems
Stolons, as mentioned above, are modified stems of plants that participate in the asexual reproduction of many species.
In contrast to rhizomes (which are "branches" of the main roots capable of growing in various directions in the soil and producing independent plants in the immediate vicinity) and tendrils (which are simply supporting and holding structures of some plants), stolons are "creeping" stems that produce adventitious roots.
The tubers, which are also considered modifications of the stem, are actually modified stolons that instead of differentiating their apices (ends) in new plants, they widen and store reserve substances.
Stolons in animals
In the animal kingdom, stolons are expansions like “roots” that project from the body wall of some small multicellular animals. These originate "buds" that, when developing, produce new zooids capable of giving rise to complete animals that connect with each other through stolons.
They are especially important in:
- Anthozoans: colonial marine cnidarians such as anemones, corals and sea “feathers”
- Hydrozoans: cnidarians such as hydroids and hydromedusas (hydras, for example)
- Stolonifers: cnidarians that are simple polyps separated by stolons like “ribbons” that form lattices
- Sea squirts: belonging to the phylum of the chordates and which are also known as sea “syringes”
- Ectoproctos: which are sessile colonies of zooids. In stoloniferous species such as Bowerbankia sp. colonies are linked to each other by stolons
- Some hemichordates such as members of the genus Rhabdopleura, whose zooids are also interconnected by stolons
Asexual reproduction of a cnidarian (Rhizostoma luteum) by means of stolons (Source: Karen Kienberger via Wikimedia Commons)
Most of the stolons in this group of living beings contribute to the formation of colonies, since these are extensions of tissue that allow the formation of clonal individuals, multiplying the size of the populations.
The zooids that are formed from the asexual buds produced by stolons come, almost always, from a very few individuals that were the product of a sexual reproduction event, which is why the colonies are groups of genetically identical organisms.
Stolons in mushrooms
Many species of fungi reproduce asexually through stolons, but the most representative case is that of black bread mold or Rhizopus stolonifer. This species is also responsible for the rotting of many fruits and moist foods rich in caloric content (carbohydrates).
These zygomycetes can reproduce sexually and asexually and, in both cases, use spores for this purpose. Their mycelia are dispersed by means of stolons, which are specialized hyphae that are distributed on the surface of food.
Diagram of asexual reproduction by stolons of black bread mold (Source: Pancrat traduktita de Bildoj via Wikimedia Commons)
As in plants, wherever stolons come into contact with the surface they produce rhizoids for fixation and from these structures they form a vegetative body known as a sporangiophore.
The sporangiophores have sporangia at their ends, characterized by their black color and possessing asexual spores that are released to germinate in other regions of the food and continue the vegetative reproduction of the mold.
References
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