Welwitschia mirabilis - The strangest succulent plant in the world
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the strangest succulent plant in the world
There Welwitschia mirabilis it is a plant that we find in south-western Africa, in the desert areas of the Kalahari and the Namib, between Angola and Namibia. It is the only species of the order Welwitschiales and the family of Welwitschiaceae.
It is a gymnosperm (such as the cicas, the Gingko biloba, cypresses and conifers in general) and grows where there are fogs, within 25 - 120 km from the coast, along a strip of about 1000 km, which goes from the Kuiseb River, in Namibia, to Moçamedes, in Angola.
It is a perennial plant with two huge non-deciduous leaves that completely comes out of the traditional life patterns of sub-desert plants. This is because it is not a desert plant, but it is an incredible adaptation of a forest tree to an arid climate.
There Welwitschia mirabilis belongs to a group of very ancient plants that had their maximum diffusion 135-205 million years ago, when the deserts were rainforests. They can be considered to all intents and purposes the last trees of a prehistoric forest.
It was first discovered by Friedrich Martin Joseph Welwitsch (hence the name), an Austrian physician and naturalist who discovered the Welwitschia, near Cabo Negro in Angola, on September 3, 1859.
Charles Darvin called it the platypus of the vegetable kingdom (note 1).
It has a very short and massive stem and two huge ribbon-like leaves that live all their life and grow fraying in contact with the scorching desert soil.
The Welwitschia mirabilis they grow non-stop, 10-20 cm per year. They could theoretically reach one meter and a half in width and an indefinite length, but in their continuous movement the tips, touching the ground, burn and then fray and over time break into many strips along the ribs. leaves that grow perpetually. A meristematic tissue continuously produces new cells and in its long life this plant produces at least 1,000 m2 of leaves!
Each individual can live up to 2,000 years.
The leaves of the Welwitschia mirabilis they are hard, leathery, lack the waxy coating typical of many desert plants and offer the sun a huge surface, as if the plant abounded with water.
As for stomata, one might think that we have very few of them, thinking about the losses related to photosynthesis in the desert. On the contrary, they count over 250 per mm2, on both sides, more than most plants. A memory, perhaps, of the life spent, millions of years ago, in the rainforests. They are thought to have maintained this characteristic because the more stomata there are, the more water they can absorb in the morning. Then, during the day, when the air becomes very hot and dry, the stomata close. In fact the Welwitschia mirabilis adopts CAM metabolism (Crassulacean Acid Metabolism), first discovered in a group of succulents, the Crassulaceae. In practice, the plant opens its stomata at night or at dawn, when it is cool and carbon dioxide can enter without the wind and heat taking away too much water. CO2 is retained in organic acids and is subsequently transformed with the sun into sugars and starches. Amazing!
They are dioecious plants that is to say there are plants that bear only female flowers and plants that bear only male flowers.
As for pollination, this is entrusted to the wind, but in practice an insect, the Probergrothius sexpunctatus, which lives almost in symbiosis with the Welwitschia mirabilis. It spends most of its life sucking female cones, and favoring their infection by microscopic fungi, thus helping to ensure that of the theoretical 10,000-20,000 seeds per plant, only 20-200 per year are saved. The seeds, once formed, do not go very far from the mother plant but fall close to it. However, since the water and food resources are very scarce, the young plants do not compete with the mother plant and organize themselves to grow at a certain distance. In fact the seeds are covered with powerful germinative inhibitors (to remove them it takes at least 25 mm of rain, continuous or concentrated in 2-3 days) and the Probergrothius sexpunctatus, causing the death and fall of most of the seeds, almost all of them dissolve at the base of the mother plant. In short, the surrounding soil is soaked with anti-germinative substances and in this way the birth of other plants is impossible and only the seeds that have fallen away from the mother plant will be saved.
Pollination takes place between November and March. After which the female cones swell, the scales rise and the seeds are dispersed by the wind. The seeds, rich in proteins and carbohydrates, are extremely hygroscopic and can wait up to 3 years to germinate, waiting for the environmental conditions to allow it.
The roots of the Welwitschia mirabilis they stretch several meters into the ground and are as deep as the width of the plant. They perform an important reserve function and absorb, with their lateral ramifications, the water that filters into the subsoil. It has also been observed that the huge carrot-shaped roots emit toxic substances into the soil.
In short ... we are dealing with a great masterpiece of nature.
1. The platypus is considered a mix of duck, beaver, otter and scorpion: the duck for the beak and webbed legs, the beaver for the tail and the otter for the body and coat. The scorpion because the males are also poisonous. On the heels of the hind legs they have stinging spurs that they can use to attack any enemy with poison. The reproduction of the platypus is almost unique. It is one of only two mammals to lay eggs (the other is the echidna).
Online bibliographic sources: Monano Nature Encyclopedia
World succulent plant
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The strangest plants in the world
In our gardens we are used to seeing common plants, such as daisies, tulips and geraniums, and perhaps we consider strange, somewhat unusual plants such as aquatic or carnivorous ones, actually in nature there are really special and strange plants and flowers, with bizarre shapes and from the most unlikely colors we see together which are the eight strangest plants in the world.
There Rafflesia arnoldii is the plant that develops the largest flower in the world, which can reach three and a half meters in width, is red with white spots, gives off a bad smell and has a hole in the center that can hold 6 to 7 liters of water, another peculiarity of the Rafflesia it is that it has neither stem, nor leaves, nor roots as it is a parasitic plant.
Hydnora africana is characterized by a flesh-colored flower with a very particular shape and bad smell that sticks to the roots of shrub plants of the arid deserts of South Africa.
There Dracunculus vulgaris it is composed of a burgundy-colored spathe and a very long black pointed flower.
The plant which, in addition to being very strange, also has the most particular name is theAmorphophallus, which translated literally means "shapeless male genital" due to its spadix-like inflorescence that can reach 3 meters in height during flowering which lasts about 3 days, the plant, originally from the island of Sumatra, gives off a very unpleasant odor.
There Wollemia nobilis it is a very bizarre-looking tree that can reach 125 meters in height. It was only discovered in 1994 by David Noble, from whom it takes its name, despite being one of the oldest and rarest plants that exist in nature. Its main feature is that of having a bark made up of bubbles, multiple trunks and leaves that develop in a spiral.
There Welwitschia mirabilis is a plant native to Namibia, in Africa, and it is such a particular plant that Charles Darwin himself called it "the platypus of the vegetable kingdom" is composed of a root and two hairy leaves which, as they grow, take on very strange shapes, and which can stretch up to 5 meters in length. This plant can live very long, just think that specimens of over 2000 years of age have been found.
There Drakaea glyptodon it is an orchid originally from Australia, also called "hammer orchid" its peculiar characteristic is given by the fact that both the color and the smell resemble raw meat and for this reason it attracts insects for pollination.
There Wolffia cramped produces the smallest flower in the world, just think that a dozen plants can fit comfortably on the head of a pin is an aquatic plant native to Asia.
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“Welwitschia”, the plant that lives up to two thousand years
Native to southwestern Africa, it grows in desert areas. In Pavia there is one (younger) in the Serra Scopoli
Among the miracles that nature can do, there is also that of generating plants that live for millennia. This is the case of the Welwitschia mirabilis Hook.f., plant native to southwestern Africa (born in the desert areas of Kalahari and Namib, between Angola and Namibia, in one of the most inhospitable areas on earth), which owes its name to Friedrich Welwitsch, the Austrian botanist which was the first to document its existence within the European scientific community.
Not even the adjective "mirabilis" that accompanies its name is accidental, because to characterize this plant, in addition to longevity (in the Nabib desert there are specimens that are two thousand years old), they are an unusual shape, starting with the stubby and robust trunk, to continue with the two leaves, the only ones, which at first sight resemble a green ribbon and which never die, because their life cycle is continuous: they develop throughout the life of the plant, both in width and in length, drying at the tip and gradually wrapping around the stem. In this way, being "rough", the leaves retain moisture in the soil directly around the stem and roots.
As a division, the Welwitschia it belongs to that of gymnosperms, as for the species it is "dioecious", ie composed of specimens with male flowers and specimens with female flowers, characterized by conical formation and nectar-producing ends, similar to small pine cones.
In our part, the Welwitschia it is quite rare, it is found only in some botanical gardens - including that of the University of Pavia, which houses a specimen of about thirty years old in the Serra Scopoli - and is mostly of interest to collectors and botanists (as well as tourist, for those who visit the desert), also because the conditions for its maintenance and reproduction are rather "demanding". Difficult to germinate from seed, in fact, the Welwitschia it is not easily found in nurseries, because, in fact, it is known as a "strange" plant: it can be considered a tree because it has a very woody stem, which over time expands in width more than in height, but it is not advisable to keep in the garden it has a tap root that it needs to anchor itself well to the ground and, in the desert areas where it lives, to get enough water for survival (it is able to survive in places where there is practically no rain, or, often, in depressed desert areas, where the little rain that falls drains away), so it needs a large free space around it.
Finally, due to its longevity, it can be considered a living fossil, as established by carbon dating studies, following which, the Welwitschia she was consecrated as "a creature capable of surviving time". -
La belle verte
Ribbons against drought
Ribbons against drought
Plants adopt various strategies to combat drought, which they inevitably face at certain times of the year, either due to lack of water, or due to excess radiation and therefore transpiration. There are plants capable of changing their water content based on environmental fluctuations (poikylohydric) and others whose water level does not vary much in relation to the external environment (homeodic). The plants most tolerant to drought are generally those of the first group because they have a series of adaptations aimed at tolerating the lack of water. These are mainly algae, lichens, mosses and liverworts, all plants without conductive vessels. The latter, on the other hand, are more than anything else able to avoid drought with a series of strategies linked to the condition of homehydricity. The tendency to tolerate drought becomes less and less marked as the evolutionary scale rises, in proportion to the increase in the ability to avoid it.
In an old paper, Gaff (1972) tried to list and group ferns, gymnosperms, and angiosperms (monocots and dicots) based on their drought tolerance. The most tolerant were the angiosperms, followed by ferns and gymnosperms. Now, thinking of firs, pines, larches and company, we certainly struggle to think that they are plants adapted to little water, and in fact they are not. Gymnosperms (conifers), seedless plants, are adapted to the high levels of radiation typical of mountain or cold environments, but they are very sensitive to water levels in the soil.
As with any human-based classification, there are due exceptions. One of these is Welwitschia admirables, endemic to the desert of Namibia, in south-east Africa a strange gymnosperm that has very few characteristics normally associated with plants that live in arid environments (xerophytes).
Let's get straight to the point: this plant is a living paradox.
It also has a very strange shape, with huge and very long leaves. It can live up to 2,000 years in the desert while maintaining a shape that resembles a pile of dry, brown leaves. It has a constantly crumpled and crumpled appearance, collapsed on the scorching desert ground. We will return to the structure of this plant later. For now, we are told that the leaves resist water decreases of 44% compared to maximum saturation, a high value but which does not spare them from adopting mechanisms to avoid drought. It therefore seems that this plant, more than tolerating stress, you avoid it.
The problem is figuring out how. Probably, I thought, Welwitschia it has a very low transpiration, a thick waxy cuticle, dense and / or deep roots, the ability to absorb water vapor from the fog. In fact, the cuticle of Welwitschia it is less thick (6 microns) than other plants adapted to drought, such as Euphorbia is Aloe. The latter two genera include only succulents (succulents) while Welwitschia it's a sclerophyte, with little inland water, which makes the mystery of its long desert survival even more intriguing. As for the fog, in the deserts of Namibia this is frequent in the morning hours and could cool the leaves (and thus decrease transpiration) as well as allow the droplets of water to enter the plant from the stomata present on the leaves. Finally, the superficial roots and not at all dense with Welwitschia suggest that the water vapor could also be absorbed by the roots themselves, even if this water would still be insufficient for the needs of the plant.
Not even the metabolism of Welwitschia it doesn't help to give us a plausible explanation. This species in fact does not have a CAM metabolism (crassulacean acid metabolism), typical of succulent plants, where the stomata are closed at night, when transpiration is lower and relative humidity higher, and carbon dioxide is fixed in the dark. In Welwitschia, the stomata are open mainly during the day and at night (there is a certain tendency to CAM metabolism, which is "activated" only in certain environmental conditions and in the presence of other species, but is not frequent in any case). The leaves therefore lose large amounts of water during hot days. CAM plants are by nature adapted to desert climates, and it is therefore surprising that Welwitschia, which lives in an environment that would favor CAM metabolism, behaves not as a succulent species but as a plant adapted to temperate climates. It has even been hypothesized that this plant probably originated in tropical conditions and then, with the climatic changes that occurred in the past, it gradually adapted to desert conditions. This too, at least to me, seems like an explanation that doesn't stand up very well.
I have read a lot of bibliography on Welwitschia but it hasn't given me many clues as to its unusual drought tolerance. The works were all quite old from the 1980s onwards, the species has no longer been studied and has remained a botanical curiosity. This curiosity derives above all from the appearance of Welwitschia, so I decided to start from this to understand something.
I start from roots, which should give us some important clues. The roots are superficial: they reach an average depth of 1.5 meters (maximum 3 meters) not even at the water table. The first 25 cm of roots are also heavily lignified, which not only avoids water loss but also makes the roots a reservoir of water. There are a series of lateral roots and a network of delicate spongiform roots. The total length of the root system is modest compared to that of other desert plants. The root apex has a telescope shape, with cells that are very metabolically active and rich in starch. An unusual fact is that these cells contain droplets of oils and other fats (indicated by G and F, respectively, in the figure below) but I cannot associate fat metabolism with the water state of the plant except for the fact that from the combustion of 1 g of fat about 1 mL of metabolic water is produced (a little bit for the needs of a plant).
All these details on the roots give me the impression of a very normal plant typical of temperate climates. At this point, I thought the solution lay in the leaves.
The two leaves, opposed to each other and permanent, are the most striking, surprising and anomalous feature of Welwitschia mirabilis, hence the common English name “two-leafed-cannot-die”. They are ribbon-like and spread out like a torn and tangled mass over the desert gravel, giving the plant a strange and grotesque appearance. The plant resembles a tangled octopus on the desert. The leaves are ribbon-like and continuously perpetuated at a rate of 8-15 cm per year by an intercalary basal meristem (this type of meristem together with the parallel vein of the leaf are also characteristic of monocotyledons). In younger plants, the two opposite leaves grow and curve outwards, away from the stem, touching the sand after 0.3-1.2 m. La parte terminale della foglia, che si trova a contatto con la sabbia, viene praticamente arrostita. I venti caldissimi del deserto, inoltre, scuotono le le foglie, smuovendo le loro parti terminali avanti e indietro sulla ghiaia, con il risultato che le punte delle foglie si sfilacciano e si consumano progressivamente man mano che la crescita avviene. The foglie possono avere dimensioni enormi: larghe fino a 2 m, lunghe fino a 8 m, con una superficie fogliare totale (delle due foglie) fino a 20 m 2 di tessuto vivente e fotosintetico. Le piante giovani hanno misure più ragionevoli ma pur sempre notevoli (0.5-1.0 m di larghezza e 1-2 m di lunghezza).
Welwitschia mirabilis, orto botanico di Berlino (foto mie).
Quello rimane poco chiaro è il significato di tali grandi superfici fogliari, perché la sopravvivenza stessa di una pianta del deserto dipende dalla sua capacità di ridurre la superficie delle sue foglie al fine di contrastare la perdita di acqua per evaporazione e traspirazione. La risposta risiede in parte nella nebbia costiera dei deserti namibiani, che si sposta nell’entroterra prima dell’alba e scompare alle 10:00 circa. There condensazione della nebbia, che in termini di precipitazioni è equivalente a una pioggia annuale di 50 mm, si raccoglie sulle foglie, viene assorbita attraverso gli stomi e condotta in varie parti della pianta attraverso un ampio sistema di vasi. Non è quindi sorprendente scoprire che la foglia possiede milioni di stomi su entrambe le superfici, superiore e inferiore (più di 20.000 per cm 2 ). Le piante xerofite, per non perdere acqua, normalmente non hanno stomi sulla superficie fogliare superiore e quelli sulla superficie inferiore sono pochi e profondamente infossati. In effetti, gli stomi di Welwitschia si chiudono poco dopo che la nebbia scompare. Gli stomi sono infossati, ma non così profondamente come nella maggior parte delle xerofite. Tuttavia, possono chiudersi ermeticamente e sono invariabilmente supportati da celle meccaniche enormi, estremamente potenti. Queste caratteristiche fogliari suggeriscono un notevole adattamento complessivo alle condizioni di un deserto con nebbia frequente. Nonostante questi accorgimenti, è stato dimostrato che molta dell’acqua condensata viene comunque persa e non assorbita dagli stomi.
Rimane però da capire se una pianta del genere possa sopravvivere con solo 50 mm di acqua annuali. E ancora, molte piante grasse desertiche hanno strutture chiamate idatodi, delle particolari forme di stomi in grado di assorbire acqua dall’aria, ma questi non sono presenti in Welwitschia. Ancora una volta, Welwitschia appare una pianta normalissima ma con una grande capacità di fronteggiare la siccità.
L’epidermide della foglia di Welwitschia è coperta da una cuticola cerosa costituita da tre strati, che costituiscono solo 1/40 dello spessore totale della foglia. Tale dato è piuttosto sorprendente, dal momento che la cuticola di specie del deserto di solito è molto più spessa. Tuttavia, lo strato centrale della cuticola è il più spesso e contiene cristalli di ossalato di calcio che probabilmente hanno un ruolo importante nel riflettere l’energia radiante proveniente dal sole, e di conseguenza nel mantenere la foglia fresca. In effetti, solo il 55% della radiazione solare è assorbito dalle foglie di Welwitschia e la restante parte è riflessa (in molti alberi, in media solo il 25% della radiazione è riflessa). Riflettere la luce, e quindi anche il calore, è una delle migliori strategie che una pianta desertica con grandi foglie abbia per conservare acqua.
Andando un po’ più nel dettaglio, la foglia è ricca di fibre affusolate molto lunghe (sclereidi), che la rendono rigida ma flessibile allo stesso tempo, analoga a una barra di acciaio ripiegata. La foglia è sufficientemente rigida da formare una parte curva di considerevole lunghezza, prima di toccare la sabbia. È qui comincia il bello: la parte della foglia non a contatto con la sabbia ha una temperatura di 4-6 °C maggiore di quella dell’aria. Questo perché nella zona ricurva delle foglie, l’alta riflettanza (55%) minimizza l’assorbimento da parte della foglia della radiazione solare a corta lunghezza d’onda incidente dall’alto allo stesso tempo, il suolo relativamente freddo sotto la parte ricurva della foglia minimizza l’assorbimento della radiazione infrarossa a lunga lunghezza d’onda emessa dal suolo. Attraverso questi due meccanismi, la foglia evita il surriscaldamento. D’altro canto, la parte della foglia che tocca il terreno ha temperature più alte perché lo scambio di calore sulla superficie inferiore fogliare (emissione di infrarossi) è ostacolato.
Questa figura in alto (Lambers et al., 2008), che spiega quanto appena detto solo in termini di scambi di calore, non approfondisce però il discorso dell’acqua, cioè il punto cruciale, nonché fattore limitante per Welwitschia. La figura somiglia incredibilmente a quest’altra, emersa dai miei cassetti della memoria, e pubblicata nel Terzo Manuale delle Giovani Marmotte, fortunatamente ancora nelle mie mani dopo 40 anni.
L’articolo (“Per trovare l’acqua – dove non c’è – con l’alambicco solare”, pp. 38-39) descrive un metodo per recuperare fino a 1,5 litri di acqua al giorno facendo evaporare l’acqua da una buca scavata nel terreno (diametro di 1 m e profondità di mezzo metro), lasciandola condensare sotto un telo di plastica trasparente ricurvo di 1,5 m di lunghezza, e poi raccolta in un secchio.
In termini di dimensioni, siamo là. Immaginiamo che il telo di plastica sia la parte ricurva della foglia di Welwitschia, il secchio l’insieme degli stomi aperti nella pagina inferiore della foglia, e la buca nel terreno (concava) la zona sottostante alla parte ricurva della foglia (convessa). La differenza di temperatura suolo-telo, nella figura dovuta alla buca, è equivalente a quella della differenza suolo ombreggiato-foglia. Il discorso quadra perfettamente. Il segreto di Welwitschia è proprio nella foglia. È per questo che ci sono stomi nella parte superiore e inferiore. Dai primi viene assorbita l’acqua condensata dalla nebbia, dai secondi quella del suolo. La foglia Welwitschia è un alambicco solare, che usa le differenze di temperatura foglia-atmosfera e foglia-suolo per condensare acqua e poi assorbirla dagli stomi e trasportarla al resto della pianta.
Del resto, come insegnano Qui, Quo e Qua, il Manuale delle Giovani Marmotte è per definizione infallibile e dalle infinite potenzialità.
Grazie a loro, ho scritto:
Bornman CH (1972) Welwitschia mirabilis: paradox of the Namib Desert. Endeavour, Volume XXXI, Welwitschia mirabilis: paradox of the Namib Desert 113: 95-99.
Bornman CH, Elsworthy JA, ButIer V, Botha C.E.J. (1972) Welwitschia mirabilis: Observations on general habit, seed, seedling, and leaf characteristics. Madoqua, series II, Vol. J, nos. 54-62: 53-66.
Disney W (1977) 3° Manuale delle Giovani Marmotte. Arnoldo Mondadori Editore.
Gaff DF (1972) Drought Resistance in Welwitschia mirabilis Hook. fil. Dinteria 7: 3-7.
Lambers H, Stuart Chapin III F, Pons TL (2008) Plant Physiological Ecology. Springer.
Schulze E-D, Ziegler H, Stichler W (1976) Environmental Control of Crassulacean Acid Metabolism in Welwitschia mirabilis Hook. Fil. in Its Range of Natural Distribution in the Namib Desert. Oecologia (Berl.) 24: 323-334.