Water is a limited resource in desert landscapes, yet plants such as cacti thrived in the barren region. How do they survive several months of drought?
Cacti store water in their stems and enlarged roots that contain collapsible water storage cells called parenchyma. The cells have flexible walls that expand and shrink depending on the water level in the plant. Water is collected through the roots, stems, and spines.
Cacti growing in the wild have adapted a lot of features to sustain life in the desert. One of their adaptations is to store and conserve water in their stem and roots. How can that be? Well, let’s discover how they collect and store water from nature.
Cacti collect water through their extensive root systems, specialized stems, and conical spines. They have developed amazing water management tactics to absorb and conserve as much water as possible.
According to Earth Observatory, most desert biomes receive very little rainfall (about 10 inches) per year. But if it does rain, it can be quite a heavy downpour. Given these diverse desert conditions, cacti need to evolve and adapt to gathering and saving water. Here are the parts of the cactus responsible for collecting water.
Many cacti have extensive shallow roots growing just a few feet deep but extending, branching several feet away from the base and surrounding the plant. This characteristic enables them to have easy access to the water as soon as it falls into the ground.
Rain seldom penetrates a few inches into the thirsty desert soil. Cacti roots close to the surface allow them to quickly soak and take up rainwater, increasing the absorption rate.
Aside from their shallow spreading roots, some cacti have adapted deeper taproots to anchor them and obtain water from the underground. Moreover, cacti roots have the unique ability to shoot out tiny root hairs or trichomes when the soil gets wet to aid with the collection of moisture.
Cacti have specialized stems containing tiny pores called stomata that open only at night. When the pores open, the stomata take in moisture available in the air along with carbon dioxide.
During the nighttime, humidity in the desert is high, and water vapor can be abundant. Cacti adaptation to open their stomata only at night helps with the collection of water from the air. Although the stem does not absorb moisture as much as the roots, it can help add to the water reservoir.
Cacti develop a strategy to collect water through their long conical spines. During the cold nights in the desert, mists in the air are converted into water droplets. These droplets adhered to the conical spines and were collected.
The water droplets collected from the spines were directed towards the open stomata and absorbed in the stem. Some dewdrops run down the stem and fall on the ground, which the roots take up too. This strategy applies during rainfall as well.
All cacti store water in their collapsible water storage cells found inside their fleshy stems. These cells are also present in the roots of some cacti growing in the mostly dry regions. They have modified root systems that store water and retain them for quite a long time.
Beneath the cacti epidermis are layers of tissues that carry out vital functions for the plant’s survival. The outer layer is composed of chlorenchyma and mucilage cells. Photosynthesis is achieved by the chlorenchyma cells, so they are the ones that need water the most.
In cacti, the primary parenchyma cells are in-charge of storing a large amount of water in the stem or roots to keep the plant hydrated during dry spells. They have flexible cell walls that expand when there is enough water intake and collapse when water is released to other cells.
Because it contains almost water, the innermost layer of the stem is succulent and soft, making people think living cacti are hollow. During the heavy outpour, the roots gather water and transport them up to the stem, filling the outer layer cells first, then the innermost tissues.
Cacti will drink up as much water as possible until every single collapsible cell in the stem is filled up. The water is stored until drought occurs, and water volume in the stem reduces. When this happens, the flexible cell in the stem releases water for redistribution to other dried plant cells.
Consequently, the water storage tissues provide water to the chlorenchyma cells so that photosynthesis will continue.
Other cacti growing in the driest regions develop an enlarged stem to store water and nutrients. Lophophora williamsii or peyote cactus, Ariocarpus, Echinocereus, and some Mammillarias are examples of cacti with tuberous roots.
Because cacti do not have the luxury of water in the desert, they need to carefully manage every available drop of water to survive. Aside from having extensive roots and specialized water storage stem cells, desert cacti have adapted a few more tactics to limit water loss.
Some other traits that cacti have developed are:
- Waxy skin: Their thick outer skin helps them prevent the water inside from evaporating and keep the plant tissue cool by reflecting the hot sun rays.
- Dense Spines: Spines covering most of their stems hinder the hot airflow from reaching the epidermis and keep transpiration low. It also helps protect the plant from consumers that would want to drink their water.
- Pores or stomata opening only at night: Stomata being active at night and close during the day lessen the risk of water escaping from the pores.
- Multiple pleats in the stem: The folds in their stem called ribs appear like an accordion that expands when water is absorbed and shrinks when dehydrated. This ability provides maximum water storage.
- Falling off of root hairs: During drought, the cactus is more hydrated than the substrate it is growing. The plant purposely breaks off tiny roots where water tends to escape on the ground.
A fully-grown cactus can store an amount of water that varies greatly between 10 and 100 gallons. The exact amount of water a cactus can hold depends on its size and the number of ribs that will expand upon water absorption.
The bigger the cactus, the higher the volume of water it can retain on its stem. Also, the higher the number of ribs around the cactus stem, the more water it can hold.
According to Washington State University, a giant Carnegiea gigantea ‘Saguaro Cactus’ that stands 300 feet tall can store 4,800 pounds (95 gallons) of water in its stem during heavy rainfall. However, a full-grown 3ft wide Golden Barrel Cactus can stash up to 20 gallons of water.
The length of time the cacti store water is dependent on their species, the environment they’re growing in, and their size. The cooler the temperature, the less water the plant can use. Also, the smaller the cacti, the faster it can use up its stored water.
Cactus does not use up much of its water during winter and can go three to four months without watering. Some go dormant during this time and do not absorb any water within the duration of the cold season. That is why it is recommended to cut back from watering in wintertime.
Tiny cactus seedlings are kept inside the greenhouse to lessen the possibility of dehydration. Since they only have small stems, they can only store a little water and need constant hydration.
In general, cactus water is not safe to drink as it contains alkaloids and acids harmful to humans and pets. However, there are some species like Opuntia or prickly pear that have a low concentration of chemicals and so their stored water is safer.
Native groups in America have been using the prickly pear as their source of food. They harvest the younger pads and remove the spines before cooking. Some use a food processor to extract the liquid to make cactus juice. Opuntia fruit is edible, as well as Hylocereus “Pittaya” fruit.
- Cactus store water in their stems and enlarged roots with the help of collapsible water storage cells called parenchyma cells.
- Most cacti developed several adaptations to sustain life in the inhospitable desert. They have evolved spines instead of leaves, grow extensive roots to take up more water, succulent stems to store water, and allow stomata to open only at night.
- Most cacti water taste acrid and is not recommended to drink due to the alkaloids and acid substances. Opuntia cactus, however, is an exception as it has unconcentrated chemical content.
- “Desert Biomes,” Earth Observatory
- “How To Transplant A Cactus,” University of Arizona
- “Plant Cells- Tissue and Tissues systems,” Furman University
- “A multi-structural and multi-functional integrated fog collection system in cactus,” by Jie Ju, Hao Bai, Yongmei Zheng, Tianyi Zhao, Ruochen Fang, and Lei Jianga, National Center For Biotechnology Institute
- “Chlorenchyma in Cactus,” Cactus Experts Organization
- “Mucilage cell, cactus,” University of Texas
- “How do cacti survive in such hot and dry environments?” Washington State University
- “Prickly Pear Cactus: Food of the Desert,” by Hope Wilson, Melissa Wyatt, Patricia Zilliox, Arizona State University
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