Stem Adaptations
Due to the unique ecological niche of hydrophytes, they have many stem adaptations which differ to the other plant groups. Firstly, hydrophytes are not lignified, unlike mesophytes and xerophytes. Since hydrophytes live in an aquatic habitat, any extensive water movement may cause these plants to break if they were lignified. This adaptation may cause some limitations, as the plant loses support, but are able to overcome this as a result of the habitat they are found in.
Hydrophytes have reduced vascular and support tissues [6]. This is because these plants rely on the water for any necessary support and buoyancy, and not the phloem and xylem cells hence do not require lignin. The vascular system of these plants is also not needed. Often, the xylem tissue of hydrophytes is very poorly developed. Due to the ecological niche of these plants, water can enter through direct diffusion, so there is no need for the xylem tube. However, the limitation of a heavily reduced vascular system means that if the environment changes, or if the plants are removed from the water, they will wilt rapidly, even when the stem has access to water, resulting in the reduced chances of survival if hydrophytes were put in these situations [19].
An adaptation that is absent in xerophytes and mesophytes, but is found in hydrophytes, is a hollow stem. This adaptation in due to hydrophytes living in aquatic environments, and having a different ecological niche to the other plant groups. By having a hollow stem, this produces more air space in the plant, thus provides more buoyancy, which is needed especially if the plant is not fully submerged.
Unlike hydrophytes, it is common for mesophytes to have substantially developed stem and xylem. As a result of the environment they are found in, and the conditions their habitat may experience, mesophytes require support to stay upright, and also must withstand high winds that can sometimes occur. Since water is transported from the roots and into the plant, the xylem system must be developed for water to travel through the plant and into the leaves, which is where water is most needed for photosynthesis [7]. A developed xylem system also allows for transpiration pull to occur, and this is an essential process which must occur in mesophytes, so water can reach the leaves and so the plant is able to carry out photosynthesis, and therefore its life processes, in order to survive.
Hydrophytes have reduced vascular and support tissues [6]. This is because these plants rely on the water for any necessary support and buoyancy, and not the phloem and xylem cells hence do not require lignin. The vascular system of these plants is also not needed. Often, the xylem tissue of hydrophytes is very poorly developed. Due to the ecological niche of these plants, water can enter through direct diffusion, so there is no need for the xylem tube. However, the limitation of a heavily reduced vascular system means that if the environment changes, or if the plants are removed from the water, they will wilt rapidly, even when the stem has access to water, resulting in the reduced chances of survival if hydrophytes were put in these situations [19].
An adaptation that is absent in xerophytes and mesophytes, but is found in hydrophytes, is a hollow stem. This adaptation in due to hydrophytes living in aquatic environments, and having a different ecological niche to the other plant groups. By having a hollow stem, this produces more air space in the plant, thus provides more buoyancy, which is needed especially if the plant is not fully submerged.
Unlike hydrophytes, it is common for mesophytes to have substantially developed stem and xylem. As a result of the environment they are found in, and the conditions their habitat may experience, mesophytes require support to stay upright, and also must withstand high winds that can sometimes occur. Since water is transported from the roots and into the plant, the xylem system must be developed for water to travel through the plant and into the leaves, which is where water is most needed for photosynthesis [7]. A developed xylem system also allows for transpiration pull to occur, and this is an essential process which must occur in mesophytes, so water can reach the leaves and so the plant is able to carry out photosynthesis, and therefore its life processes, in order to survive.
In xerophytes, however, a large amount of tissue can be found. This is because a large amount of tissue allows for the plant to take in as much water as possible on occasions when rainfall does occur. When water is absorbed by the plant, the tissue becomes very succulent, and holds a water reservoir which can be crucial to the plants survival, as water is essential, but is also scarce. The stem of xerophytes often contains woody supports, which allows for the plants to remain upright, especially as water is scarce in the conditions of their habitat. The succulent tissue many xerophytes possess means that having a strong, rigid, and lignified stem ensures that there is enough support provided to ensure the plant can stay upright and survive, as well as being able to store a supply of water due to the arid conditions of their habitat.
In some xerophytes, the stem consists of ridges. These ridges will allow for the expansion of the plant, so that the maximum amount of water can be absorbed and stored on the rare occasion when a large amount of rainfall occurs. Some xerophytes have adapted to store water in its stem to support the plants water balance, like the baobab tree, which has the ability to store water in its trunk, in order to survive harsh droughts that can occur. Unlike mesophytes and hydrophytes, the stomata are found on the stem of some xerophytes [2]. This is because if the xerophyte has adapted to carry out photosynthesis in its stem, and in photosynthesis the gas exchange of carbon dioxide in and oxygen out is necessary. Therefore, the plant will have adapted to grow stomata on the stem.