Xerophilic plants or desert plants
Very thick leaves that in some cases tend to curl, thick stems and a handful of particular contours make these plants the queens of the desert. We are talking about xerophilic plants that are also known as desert plants because that is one of their natural habitats.
They are plants that develop in adverse conditions, with arid and dry climates that require special adaptation. Read on to discover what these are and how they have adapted to the environment to survive.
What are xerophilic plants?
Xerophytic or simply xerophytic plants are specialized plants, that is, plants that throughout their evolution have developed adaptation mechanisms in order to withstand the adverse conditions in which they live, such as the typical aridity of deserts or the difficulty in getting water growing on a tree.
This is how over time and as they evolved the different species have become resistant from certain changes. The funny thing is that they can belong to different families that are not related, since the only trait they share is precisely this ability to adapt. That is why they solve the problems of the environment that surrounds them by developing various strategies. We find xerophilous plants in groups as distant as ferns and allies, cycads, gnets, conifers, and a large number of monocots and dicots.
What are xerophilic plants?
As we have said before, we find xerophytes in all types of plants, since it is only the species that have adapted to drought. Since it is impossible to make a list with all the species that exist, we are going to separate them into several groups:
Succulent plants
Image – Flickr/ Pamla J. Eisenberg
They are simply plants that accumulate a lot of water in their tissues. In this way they can usually go a long time without a supply of water, but this is not always true. Although the vast majority of succulents are xerophilic, not all are. For example, Crassula aquatica is a succulent plant that lives in humid areas, and belongs to the main family of succulents, Crassulaceae. The genus Salicornia are also succulent plants that live associated with water, but in this case with salt water (halophytes). Among those that are xerophytes we find again, plants that are not related at all:
Succulent monocots
Image – Wikimedia/ Rod Waddington from Kergunyah, Australia
Within the monocots we can find the Asparagaceae family, the main family of monocotyledonous succulents (formed almost exclusively by xerophilous plants) where we find dragon trees ( Dracaena spp. ), Yuccas ( Yucca spp. ), Agaves (Agave spp.) And other similar plants, as well as other plants with the buried succulent part, such as asparagus ( Asparagus spp. ) with its tuberous roots or some bulbous ones.
Another important family of succulents is Xanthorrhoeaceae (that of aloes and asphodel). In the rest of the monocot families, the succulent part tends to be buried, so there is discussion about whether they should be considered succulents or not. In others, such as Musaceae or Araceae, what they have is an airy parenchyma that accumulates water-saturated air, so there is also discussion, but they are not xerophytes.
Succulent dicotyledons
Within the dicotyledons we have the famous cacti ( Cactaceae family ), the crassulaceae ( Crassulaceae family ), the euphorbias ( Euphorbia genus, although there are succulent species and others that are not), living stones, cat’s claw and other aizoaceae (Aizoaceae) and many other families with individual species that are succulent, such as Senecia and the like (part of Asteraceae ), Asclepiadoides, Pachypodium and Adenium ( Apocynaceae ), etc.
Other succulents
We also find a few succulents among gymnosperms (plants with seeds but without fruit), specifically the genus Ephedra and some cycads. Although these plants are often ignored when talking about succulents, as xerophytes they are without a doubt, we will talk about them later.
Halophytes
These are plants adapted to living in areas with a high amount of salt. This makes many people think that they also endure drought, but this does not have to be the case. Of course there are halophytic plants that are also xerophytes, such as Atriplex halimus, Suaeda vera, Tamarix spp., Astrophytum asterias or a large part of the plants of coastal dune ecosystems, but it is important not to confuse halophyte with xerophyte since the adaptations are different, although they can present both at the same time. Halophytes tend to accumulate salt, which raises their osmotic pressure and allows them to absorb salt water, for which they need mechanisms that allow them to expel it without adding it to their metabolism. In most cases this requires a continuous supply of water, as in mangroves or salicornia. Many of these plants are also succulent but do not tolerate drought.
Epiphytes
The epiphytes are plants that grow on other trees but get water for their own. This implies that they need adaptations to obtain water from the air. The best known epiphytes are the bromeliads, which have two main adaptations: the large ones place the leaves in the shape of a glass to accumulate rainwater and use it up little by little. The smaller ones (Tillandsia spp.) Have hairs on the leaves that capture water from the environment. Curiously, this adaptation allows two species of this genus to live in the Atacama Desert, the driest place on the planet.
Other epiphytes such as mosses go dormant when there is no water present, activating only when they get wet. A very interesting and unique adaptation is that of the Myrmecodia or ant plant, which forms galleries in its thick stem for the ants to use and hydrates itself thanks to its excretions and respiration.
Many others simply obtain their water from the moisture that accumulates in the trunk of the tree in which they are growing and do not need to be xerophytes. In drier climates where there are no epiphytes, it is not difficult to see a xerophyte such as a fig tree, a prickly pear or an agave growing in a hollow in the trunk of a tree.
Monocotyledons
As we have said before, within the monocots there are numerous succulent plants (although not all of them are xerophilic plants), however, there are also many species of xerophytes that are not succulents.
The most typical examples are palm trees from dry climates, such as almost the entire genus Phoenix (where we find the Canary Island and date palms ), the European palmetto ( Chamaerops humilis ), the washingtonias ( Washingtonia rubusta and Washingtonia filifera ), Nannorrhops ritchieana, Hyphaene spp., Etc.
Most of the grasses ( Poaceae family ) are also xerophytes, although they are perfectly adapted to grow also in flooded areas. The main adaptation of many of them is to grow and bear fruit fast, in just the couple of months that they have enough water at their disposal. Others are directly perennial and have a metabolism that allows them to consume very little water, such as grass, pennisetum or feather dusters ( Cortaderia spp. ).
Dicotyledons
In addition to succulents, here we also find numerous non-succulent xerophytes, such as a large part of Fabaceae (legumes, such as acacias, false acacias and many Mediterranean and desert shrubs ), Lamiaceae (rosemary, thyme…), Asteraceae (santolina, daisies, sunflowers,…), Malvaceae (mallow, baobabs…), Fagaceae (holm oaks, kermes oak, oak…), Apiaceae (parsley and other umbelliferous), etc. All the Mediterranean scrub and much of the Mediterranean forest (excluding pine trees, which are conifers) is made up of xerophilic dicots.
Gymnosperms
Here we find the most diverse plants. The main groups of gymnosperms are conifers, cycads, gnetids, and Ginkgo biloba. We found xerophilic plants in the first three.
Within conifers, the most important xerophytes are found in the Pinaceae and Cupressaceae families, being pines and cypresses respectively. Pines ( Pinus spp. ) Tend to be all more or less xerophilous, with their needle-shaped and rigid leaves to reduce evaporation. In general, stiff-leaved pines tolerate drought better than those with hanging leaves, but this is not always the case. The main adaptations of cupresáceas are the leaves reduced to scales to minimize the evaporation surface and the deep roots, although only some species are xerophytes, such as almost the entire genus Cupressus (cypresses), the genus Juniperus (junipers and junipers) and some moreOthers such as redwoods (Sequoia, Metasequia and Sequoiadendron), bald cypresses (Taxodium spp.) and sugis (Cryptomeria japonica) are plants that need a constant supply of water.
In cycads we find mainly tropical plants, with an intermediate aspect between a fern and a palm tree. Some of them, like the genus Encephalartos, endure a great deal of drought with their stiff leaves with spiny leaflets (they are always compound leaf) and their thick trunks filled with water. They can be considered succulent.
Gnetids are a fairly diverse and rare group made up of the single climbing gymnosperm ( Gnetum spp.), The only plant that grows by elongating its only two leaves ( Welwitschia mirabilis ), and the joint grass (Ephedra spp.). Only the last two are xerophytes, and ephedras can also be considered succulent. Welwitschia is a true desert plant, being endemic to the Namib desert, where it gets water thanks to the dew that accumulates on its two large leaves. In the case of Ephedra, they are associated with the Mediterranean area, although we can also find them in some deserts and semi-deserts of North America. It has no leaves, and has green stems that grow in segments.
Spore-producing plants
These types of plants (ferns, mosses…) are almost always associated with very humid areas, but curiously there are species that grow in deserts. Even those that live in these places are unable to retain water. This forces them to be in a state of rest until the rains come. The typical example is the false rose of Jericho, Selaginella lepidophylla, a club moss endemic to the Chihuahuan desert. Most of these amazing plants grow in the shade of rocks, where moisture is kept longer after the rains. However, xerophilous selaginella grow on rocks, where the scorching sun hits them. Much of the fern family Pteridaceae It is composed of xerophilous plants that inhabit North American deserts.
Another perhaps less impressive example is the numerous epiphytic mosses that grow in Mediterranean forests. These plants have the same adaptation, they remain in torpor dry and apparently dead until the water arrives and they hydrate. They usually grow on the north face of the trees, where the sun does not usually shine and more moisture accumulates.
Adaptation mechanisms of xerophilic plants
To withstand drought conditions, these plants have developed various mechanisms that we can place in two groups:
Physiological adaptations
Some adaptations of xerophilic plants are physiological, such as those that have managed to reduce perspiration through the cuticle or close the stomata when the heat is excessive in order to avoid massive water losses. There is a special metabolism called CAM that is typical of succulents, although not all have it. CAM is the acronym for Crassulaceae Acid Metabolism. This name was given because it was discovered in a crassulaceae, but cacti and many other plants have it too. In short, what they achieve with this metabolism is to accumulate the energy of the sun during the day and finish photosynthesis at night. This way they do not have to open the stomata during the day, saving all the water that would evaporate.
They are also physiological adaptations those of ferns and the like, which belong in a dormant state until the rains arrive.
Morphological adaptations
In other cases, the adaptations are morphological and this is where the phenomenon is most observable as it can be seen with the naked eye. Many xerophilic plants have a characteristic morphology either in the whole plant or in some of its parts. It is common for them to have dense twigs, leaves covered with waxes or hairs and thorns to withstand extreme sun exposure. In this way, they generate a cooler and more humid area in the center of the plant. Another typical morphology is spherical or pad-like plants.that achieves the same results. Of course, this also includes the swollen leaves and/ or stems of succulents to accumulate water. Another very common adaptation is to have sclerophyllous (hard) or needle-shaped leaves to reduce evaporation.
What do you think of the xerophilic plants? With them you can build xerophilous gardens, where once the plants are adapted it is not necessary to water and all the water is used. Although for these gardens, it is best to use native plants.
