A. What a wonderful question. Due to the dry, hot climate in which they arose and currently thrive, the majority of cacti lack typical leaves.
They have evolved to store water for as long as they can while it is available in order to survive. When it rains, their enormous root system is particularly effective at pulling moisture from the ground.
A normal leaf loses a lot of water through the stomatas, which are tiny openings spread out throughout the surface of the leaf. Transpiration is the term for this water loss, which is accelerated by hotter temperatures.
Cacti have evolved by losing their leaves and developing spines instead of thorns. There is a significant distinction between the two. While spines are a type of leaf alteration, thorns are a type of stem.
Cactus spines provide a microclimate by protecting the rounded or ribbed stems from the hot desert heat.
By providing cover from predators, they also serve to protect several types of desert animals.
Others use it as food, indulging in the prickly plant portions without suffering any negative consequences. Humans who may carelessly handle, take, or smell the flowers that sporadically blossom on them are discouraged by the spines.
Here is a fascinating natural anomaly. In comparison to cacti of the same species growing in partially shaded environments, such as canyons or beneath big stones, those exposed to full light have a denser covering of spines.
Q. This spring, my lovely “Gertrude Jekyll rose” is not flourishing. The new growth appears strange. There is little growth and the leaves are distorted. I believe that I have a pesticide residue issue.
A. Although it’s technically not a residue concern, this is a herbicide/pesticide problem. You or your gardener applied a weed killer containing glyphosate too closely to the plant.
Unintentionally, the spray drifted onto the canes or the foliage. There are numerous weed herbicides having glyphosate in them, with RoundUp being the most popular.
Glyphosate cannot be absorbed by plant roots, making it safe to use underneath the canopies of the majority of plants. The leaves and other green tissue on the stems, twigs, and branches serve as the point of entry. The herbicide descends, interfering with vital plant processes and causing the plant to fall over and die.
Due to the extreme greenness of all the new and/or existing canes, roses are extremely prone. Another plant that is susceptible to injury is the camellia. The other is the crape myrtle.
By employing a cardboard plant barrier, you can successfully spray glyphosate around roses and any other plant. Spray drift is stopped by the plant shield.
As you spray, you move the barrier from plant to plant. That your rose will recover is fantastic news. The growth might not resume at all for up to two growing seasons.
Are there any cacti with leaves?
Every cactus has some modifications that encourage effective water consumption. The majority of cactiopuntias and cactoids are xerophytes, or plants that thrive in hot, dry conditions, although the earliest progenitors of modern cacti were already suited to experience periods of intermittent drought.  A few Hylocereeae and Rhipsalideae tribe cactus species have evolved to live as climbers or epiphytes, frequently in tropical forests where water conservation is less crucial.
Leaves and spines
One of the most distinctive characteristics of most cacti is the absence of any discernible leaves. Although many species of Pereskia (which is close to the primordial species from which all cacti evolved) have thickened and succulent leaves, they do have long-lasting leaves.  Succulent leaves can also be found on some long-lasting cactus species, including the opuntioid Pereskiopsis.  The surface area to volume ratio is a critical factor in water retention. The amount of water present is proportional to volume, whereas water loss is related to surface area. Thin leaves and other structures with a high surface area to volume ratio naturally lose water more quickly than thick stems and other structures with a low area to volume ratio.
Even cacti with true leaves have modified leaves called spines, demonstrating that the evolution of spines came before the loss of leaves. Despite having a large surface area to volume ratio, spines in maturity have little to no water because they are constructed of fibers that are composed of dead cells.  In some species, spines serve as camouflage and protection against herbivores, and they also help conserve water in a number of other ways. They provide a moister layer by trapping air near the cactus’ surface, which lessens evaporation and transpiration. They can offer some shade, which decreases the cactus’ surface temperature and prevents water loss. Spines can condense rainwater that drips onto the ground and is absorbed by the roots when sufficiently moist air is available, such as during fog or early morning mist. 
Young Cereus hildmannianus subsp. uruguayanus stem with ribs and a waxy coating.
The majority of cacti are stem succulents, meaning that their stems serve as the primary organ for storing water. Up to 90% of a cactus’ overall mass can be made up of water. Cacti have a wide range of stem forms. Columnar cacti’s cylindrical shape and globular cacti’s spherical shape both result in low surface-to-volume ratios, which lessen water loss and the impacts of sunshine heating. Many cactus have ribbed or fluted stems, which allow the stem to contract during dry spells and then expand when it fills with water during wet spells.  During a downpour, a mature saguaro (Carnegiea gigantea) is claimed to be capable of soaking up as much as 200 US gallons (760 l; 170 imp gal) of water.  Typically, the stem’s exterior layer has a hard cuticle that is reinforced by waxy layers to stop water loss. Many cacti’s stem color has a grayish or bluish tinge because of these layers. 
Most cacti have modifications to their stems that enable them to carry out photosynthesis in the absence of leaves. More on this is covered under the heading “Metabolism” below.
Many cacti have roots that are extensively dispersed yet only reach a small depth in the soil. In one instance, a young saguaro that was only 12 cm (4.7 in) tall had roots that were no deeper than 10 cm (4 in) and had a diameter of 2 m (7 ft).  When rain follows a drought, cacti can swiftly grow new roots. Salts are present in quite high concentrations in cacti’s root cells.  All of these modifications allow cacti to quickly absorb water during brief or mild rainfall events. Ferocactus cylindraceus is said to be able to fully hydrate in a few days after soaking in a large amount of water in just 12 hours from as little as 7 mm (0.3 in) of rainfall. 
Although the stem serves as the primary organ for retaining water in the majority of cactus, some cacti also have substantial taproots.
In the case of species like Copiapoa atacamensis, which grows in one of the driest regions of the globe, the Atacama Desert in northern Chile, they may be several times the length of the above-ground body.
At night, stomata open, allowing CO2 to enter and be stored as malic acid while allowing water vapor to exit.
Day: the stomata close, the malic acid is changed back into CO2 and utilized to create carbohydrates, and the water vapor is constrained.
For plants to perform photosynthesis, they must consume carbon dioxide (CO2). Through transpiration, they lose water while doing this. Cacti, like other succulent plant species, utilize photosynthesis to lessen this water loss. The C3 process is used by “normal” leafy plants. During the day, CO2 is continuously sucked out of the air present in spaces inside leaves and transformed first into the complex 3-phosphoglycerate, which has three carbon atoms, and subsequently into byproducts like carbohydrates. Stomata, which have the ability to open and close, are responsible for controlling the passage of air to internal regions within a plant. Since the stomata must remain open in order for photosynthesis to continue, water vapor is continuously lost. This causes plants using the C3 mechanism to lose up to 97 percent of the water absorbed through their roots.  Another issue is that as temperatures rise, the enzyme that absorbs CO2 begins to absorb more and more oxygen instead, lowering photosynthesis efficiency by up to 25%. 
Cacti and other succulent plants have developed the crassulacean acid metabolism (CAM) mechanism to get around the drawbacks of the C3 system. The stomata in complete CAM only open at night, when temperatures and water loss are at their lowest. CO2 enters the plant and is stored as organic acids inside the plant cells (in vacuoles). Only this accumulated CO2 from photosynthesis is used because the stomata are closed throughout the day. At the expense of reducing the quantity of carbon fixed from the atmosphere and so accessible for development, CAM uses water far more effectively.  In the less water-efficient CAM-cycling system, stomata open throughout the daytime just like in plants with the C3 mechanism. The plant’s stomata close at night or during times of water scarcity, allowing the CAM mechanism to store CO2 from respiration for use in photosynthesis later. There is CAM-cycling in Pereskia species. 
You can determine how much CO2 a plant absorbs during the day and at night by looking at the ratio of 14 to 13 carbon atoms that make up its isotopic signature. The majority of the Pereskia species examined using this method demonstrate some level of CAM-cycling, indicating this capability existed in the parent of all cacti. According to Pereskia leaves, CAM is only present in stems and only the C3 mechanism is present in leaves.  Recent research indicates that “significant carbon absorption is exceedingly unlikely to occur in the stem,” and Pereskia species are classified as having “C3 with inducible CAM.”  All of the photosynthesis in leafless cactus is done in the stem using complete CAM. It is unclear as of February 2012[update] whether stem-based CAM originated individually in opuntias and cactoids or all at once in the core cacti; It is well known that CAM has frequently evolved convergently. 
The stems of cactus plants have undergone numerous modifications to enable photosynthesis. The ancestors of modern cacti (except from Leuenbergeria species) started delaying the development of bark and growing stomata on their stems early in their evolutionary history. However, this was insufficient on its own, as stem photosynthesis appears to be quite low in cacti with only these modifications. Stems needed to develop features that are often only found in leaves. A hypodermal layer formed just beneath the outer epidermis, consisting of cells with thicker walls that provided mechanical support. To allow carbon dioxide to diffuse inside the cells, air gaps had to be present between them. A “spongy layer” and a “palisade layer” of plant tissue called “chlorenchyma,” which is where the majority of photosynthesis takes place, evolved in the cortex, the heart of the stem. This tissue is composed of relatively unspecialized cells that contain chloroplasts. 
Why do cacti lack leaves in favor of spines?
The prickly pear cactus found in Mexico has the potential to become a fantastic energy source in the future.
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Cacti are among my favorite plant species, and their needles undoubtedly make them distinctive. In many respects, the needles that cover cactus are essential to their survival. Since many cacti are found in dry environments, these plants need to store a lot of water in order to thrive. In fact, according to scientists, water makes up between 90 and 94 percent of a cactus plant. Cacti are ideal treats for thirsty species due to their high water content. Animals that consume cactus in the wild include quail, kangaroo rats, sheep, desert tortoises, as well as a variety of insects. In order to stop thirsty or hungry animals from eating or harming the plant, cacti contain needles.
Additionally, you could see that the color and texture of cactus spines might vary. While some spines are fluffy, others are stiff. The color of the spines can also vary, from white to gray to pink! Because of these variations in spine color and texture, cactus can more effectively blend into their surroundings (i.e. camouflage). Therefore, a second reason cacti have needles is to enable them to conceal from harmful species.
Heat is a significant issue in desert regions where there are many cacti. The needles of a cactus can offer protection from the sun so that it can thrive in these scorching temperatures. Although it may not seem like a single needle may offer much shade, several cacti species have needles that are grouped closely together. The plant is shaded by these spine clusters, which resemble small umbrellas. These spines provide shade for the cactus, which helps keep water from evaporating and causing water loss.
All in all, cactus use their spines as protective and hiding mechanisms against potential predators. Additionally, they give the plant shade, which keeps it cooler and prevents water loss.
Cactus don’t have leaves, then what do they have?
Why do cacti differ from other plants in having thorns instead of leaves and a thick, meaty stem?
The appropriate choices are A The cacti can hold more water as a result. Water would be lost because of broad leaves. In the desert, you can find thorny shrubs and cactus. Their thick, fleshy stems allow them to hold more water for a longer period of time. In addition, plants in the desert have thorns rather than leaves because big, broad leaves would make water evaporate quickly. Water moves through a plant during transpiration, and it evaporates from aerial parts including leaves, stems, and flowers.