Gnetales corresponds to an order of gymnosperm vascular plants whose family Gnetaceae is represented by a single genus (Gnetum) that groups some 43 species, many of these climbers being.
The leaves of the plants of the genus Gnetum they are very similar to those of angiosperm plants. They develop two leaves at the nodes, they are wide and have a venation with a midrib that gives rise to lateral secondary veins towards the edges of the leaves..
A distinctive feature of this order and related orders within gymnosperms is that the plants have vessels in the xylem. This allows a more efficient pathway for the movement of water, and it could be assumed that, especially the species of the genus Gnetum, were the plants that would have given rise to the evolution of angiosperms.
These plants are abundant in Asia and have some representatives in America. The wood of the arboreal species is hard and at night they emit a foul odor. Of the species of Gnetum there is no record that they are usable in dendrochronology.
Regarding the uses of these plants, some are used to make ropes, others produce edible seeds and some species are useful in Chinese medicine as anti-asthmatics..
In addition, tree species due to their rapid growth and high resistance to wind (even cyclones), are important in environmental regeneration plans and in arboriculture..
Article index
Most of the species belonging to the order gnetales are tree-climbing lianas, very characteristic of tropical forests. Some representatives of this order are trees almost 10 m high, with large leaves and very similar to the leaves of angiosperm plants..
The stems of the species of the genus Gnetum are less visible than order Ephedrales, since in the latter leaves are produced on branches or short shoots, which allows its stem to be easily observed.
The leaves of the plants of the genus Gnetum they are very similar to those of angiosperm plants, especially dicotyledonous ones. These species have two leaves at the nodes, are wide and have a venation with a middle vein from which secondary lateral veins derive towards the margins of the leaves (reticulated venation).
The reproductive structures of the gnetals (as is characteristic in gymnosperms), are within strobili or cones. Most of these species are dioecious, that is, a plant produces pollen or produces seeds but not together.
In the case of the seed-producing structures they are known as megasporangiate strobili, and in those of the pollen they are known as microsporangiate strobili. In both, the two strobili have bracts arranged in the opposite direction to the armpits, and from these grow short fertile shoots.
The strobili in this genus are compact or may either be elongated by nodes and internodes. In male strobili (microsporangiate), two fused bracts form, enclosing a microsporangiophore. Normally there are two microsporangia that are seen separately at the end of each sporophyll..
On the other hand, in megasporangiate strobili the structure called dome or collar produces 8 to 10 ovules, and each ovule has three integuments around it.
In the genre Gnetum there are no archegonia present and, at the micropylar end of the female gametophyte, there is an area without divisions but with several nuclei. At this point, each nucleus behaves like an egg cell and joins the nucleus of the male gamete, thus forming a zygote..
The other male nucleus joins a female nucleus and from this fusion the endosperm is formed. In this case, there is a double fertilization although the development of the embryo does not take place the same as in angiosperms.
The pollen grains are moved by the wind until they reach the ovules where they are attached to them through the pollen drop and from there they pass to the micro-pillar tube. This pollination process, together with the formation of seed, can last a year.
The seeds are shiny in appearance and similar to yellowish drupes..
- Kingdom: Plantae
- Phylum: Tracheophyta
- Class: Gnetopsida
- Order: Gnetales
Gnetum it has about 43 species. This genus has a particular phylogenetic importance because it has been proposed as the one that gave rise to angiosperms. However, the absence of a lipoidal substance in the pollen grain diverts the attention of the Gnetophytas as progenitors of the fruiting plants..
There are 39 pantropical species gathered in the genus Gnetum, being 37 lianas. In the Neotropics there are about 10 species, in Central and South America there are 7 species, among which the species Gnetum camporum is a native of Venezuela.
Some representative species of this order are: Gnetum acutum, Gnetum africanum, Gnetum arboreum, Gnetum cuspidatum, Gnetum globosum, Gnetum edule, Gnetum klossii, Gnetum latifolium, Gnetum luofuense, Gnetum diminutum, Gnetum venosum, among other.
In many sources the presence of a single genus (Gnetum) with about 43 species. However, in some references the gnetales can be found grouping three families: Ephedraceae, Gnetaceae and Welwitschiaceae.
Within the Ephedraceae family there is only one genus (Ephedra) that groups some 65 species. Most of its species include branched shrubs and rarely small trees, as well as some climbers, but infrequently.
These species are distributed in dry and cold areas in both the eastern and western hemispheres. In the west they are found in desert areas in the southern United States, in part of Mexico and in a large area of South America..
From plants of this genus a substance known as ephedrine is produced, which is prescribed to treat colds, relieve fever, produce sweating and as a decongestant.
For its part, the Welwitschiaceae family comprises a single, very particular species called Welwitschia mirabilis. This plant is quite rare since it grows 150 km inside the Namib desert, near the coastal zone of Angola and Namibia, and where the precipitation ranges from 0 to 100 mm per year..
Welwitschia mirabilis It has only two large permanent leaves that separate and grow until the tips die when they touch the hot desert sand.
Although the mechanisms of this plant to acquire the necessary water in these conditions are not known with certainty, it develops a root 1.5 m deep from which numerous secondary roots are formed that serve to take advantage of the little available water, a characteristic that does not is present in other desert plants.
Gnetals are found in tropical rainforests in Africa, Asia, in the northern part of South America (especially in the Amazon basin), and on islands between Australia and Asia..
These species can grow in dry seasons, where the occurrence of precipitation does not exceed 40 mm per year. On the other hand, the type of soil in which they grow ranges from acidic to neutral and can adapt to soils with low fertility.
Regarding the environment they occupy, gnetals are normally under direct sunlight, however, tolerance to shade is a characteristic that would allow them to colonize and become invasive species in new environments..
Regarding their uses, the seeds of some species can be eaten roasted or cooked, and the young leaves are also edible. Some species are used as antiasthmatics.
The species Gnetum gnemon It is cultivated to use the juvenile leaves, the bases of the cones and their fibers to make ropes. The seeds of this species are also used as food for indigenous people of the tropics.
Some tree species are recommended in environmental protection programs, especially in the regeneration of ecosystems, due to their rapid growth..
On the other hand, these species are recommended for arboriculture programs thanks to their high resistance to cyclones..
According to ethnopharmacological studies, C-glycosyl-flavones and a group of stilbene complexes have been found. Gnethole is a hydroxylated stilbene in the species Gnetum ula.
On the other hand, they also contain lignans, which are substances of the guaiacyl-syringil type, while Gnetum parviflorum It has demethylcoclaurin hydrochloride which is used as an antiasthmatic in Chinese medicine.
Gnetals can grow in a wide range of soil textures. These can be sandy, clayey, silty or combinations of them.
The species of the genus Gnetum they must remain in well-drained soils, as they are intolerant to waterlogging. On the contrary, they are species quite tolerant to drought for several months..
In this sense, these plants can easily develop only with the water coming from the rain, or be watered once a week.
Gnetal species normally grow under direct sun exposure, but have a high tolerance for shade..
These plants grow in places with tropical characteristics such as an average annual temperature between 22 and 30 ° C and a minimum temperature of 12.8 ° C.
The literature referring to the gnetales order does not offer sufficient data on the diseases or pests that may affect the species of this group..
Yet No Comments