Why is xylem greatly reduced or absent in aquatic plants?
Totally submerged
Totally submerged plants are the true water plants or hydrophytes. Because they are truly aquatic they have the greatest number of adaptations to life in water. These include:
The presence of little or no mechanical strengthening tissue in stems and leaf petioles. If these plants are removed from the water, they hang limply. They are normally supported by water all around them and so have no need of mechanical strengthening. Indeed, this would be a distinct disadvantage as it would limit flexibility in the event of changes in water level or water movements.
Submerged plants lack the external protective tissues required by land plants to limit water loss. The epidermal (outermost) layer shows very little, if any, sign of cuticle formation. All the surface cells appear to be able to absorb water, nutrients and dissolved gases directly from the surrounding water. As a result, the internal system of tubes (xylem) which normally transports water from the roots to all parts of the plant is often greatly reduced, if not absent. Thus, if these plants are removed from the water, they wilt very quickly, even if the cut stems are placed in water. This is because the normal water transport system is poorly developed. As might be expected, there are also no stomata (breathing pores) on the leaves.
Roots, which normally play a very important role in the absorption of nutrients and water from the substrate, are often also reduced and their main function is anchorage. The root hairs which function in absorption are often absent and roots themselves may be entirely dispensed with (e.g. Bladderwort).
Many species have very specialised leaf shapes. The submerged leaves are often highly dissected or divided. This has the advantage of creating a very large surface area for absorption and photosynthesis. It also minimises water resistance and hence potential damage to the leaves. Heterophylly, where leaves of different shapes are produced depending on where on the plant they are, is common. This can create great problems for identification! In many cases, the submerged leaves are totally different to floating or emergent leaves on the same plant. The emergent leaves are usually much less divided, if not entire and have a more similar internal structure to those of land plants.
Air-filled cavities often extend throughout the leaves and stems of aquatic plants, providing an internal atmosphere. Certain aquatic fly and beetle larvae have a novel adaptation which allows them to take unusual advantage of this. A sharp appendage on the end of their abdomen is used to pierce into submerged plants, giving them access to the internal air-filled cavities as their own personal oxygen source.
floating plants Floating plants are of two types: those which are rooted with floating leaves (e.g. Water Lily) and those which are not rooted in the sediment, but just float on the surface (e.g. Duckweed). Floating leaves are generally tough because they have to withstand the weather and water movement.
The green pigment-containing chloroplasts important for photosythesis are restricted to the upper surface of the leaves which are the only surface to be well lit. Stomata (breathing pores), through which gas exchange takes place in the leaf, are also found only on the upper surface of the leaf. This upper surface often has a thick waxy cuticle to repel water and help to keep the stomata open and clear. Air-filled internal cavities are also often present.
Terrestrial plants such as trees have to develop an enormous quantity of structural material in order to rise above all the other plants and collect the lion's share of the light available. Water lilies provide a neat example of a plant which has managed to do exactly the same thing, but with the minimum of structural material. Weak stems produce a massive floating canopy of leaves which dominate the local aquatic plant community just as effectively as trees dominate in a woodland. The difference lies in their external medium. Water provides all the necessary support, whereas air does not.
Totally submerged plants are the true water plants or hydrophytes. Because they are truly aquatic they have the greatest number of adaptations to life in water. These include: Submerged plants lack the external protective tissues required by land plants to limit water loss. The epidermal (outermost) layer shows very little, if any, sign of cuticle formation. All the surface cells appear to be able to absorb water, nutrients and dissolved gases directly from the surrounding water. As a result, the internal system of tubes (xylem) which normally transports water from the roots to all parts of the plant is often greatly reduced, if not absent. Thus, if these plants are removed from the water, they wilt very quickly, even if the cut stems are placed in water. This is because the normal water transport system is poorly developed. As might be expected, there are also no stomata (breathing pores) on the leaves. Roots, which normally play a very important role in the absorption of nutrients and water from the substrate, are often also reduced and their main function is anchorage. The root hairs which function in absorption are often absent and roots themselves may be entirely dispensed with (e.g. Bladderwort). Many species have very specialised leaf shapes. The submerged leaves are often highly dissected or divided. This has the advantage of creating a very large surface area for absorption and photosynthesis. It also minimises water resistance and hence potential damage to the leaves. Heterophylly, where leaves of different shapes are produced depending on where on the plant they are, is common. This can create great problems for identification! In many cases, the submerged leaves are totally different to floating or emergent leaves on the same plant. The emergent leaves are usually much less divided, if not entire and have a more similar internal structure to those of land plants. Air-filled cavities often extend throughout the leaves and stems of aquatic plants, providing an internal atmosphere. Certain aquatic fly and beetle larvae have a novel adaptation which allows them to take unusual advantage of this. A sharp appendage on the end of their abdomen is used to pierce into submerged plants, giving them access to the internal air-filled cavities as their own personal oxygen source. |
Floating plants are of two types: those which are rooted with floating leaves (e.g. Water Lily) and those which are not rooted in the sediment, but just float on the surface (e.g. Duckweed). Floating leaves are generally tough because they have to withstand the weather and water movement.
The green pigment-containing chloroplasts important for photosythesis are restricted to the upper surface of the leaves which are the only surface to be well lit. Stomata (breathing pores), through which gas exchange takes place in the leaf, are also found only on the upper surface of the leaf. This upper surface often has a thick waxy cuticle to repel water and help to keep the stomata open and clear. Air-filled internal cavities are also often present.
Terrestrial plants such as trees have to develop an enormous quantity of structural material in order to rise above all the other plants and collect the lion's share of the light available. Water lilies provide a neat example of a plant which has managed to do exactly the same thing, but with the minimum of structural material. Weak stems produce a massive floating canopy of leaves which dominate the local aquatic plant community just as effectively as trees dominate in a woodland. The difference lies in their external medium. Water provides all the necessary support, whereas air does not.
