Hydrophytes, categorized into fully submerged and partially submerged groups, are plants that reside in water. Their ability to thrive in watery environments is facilitated by various characteristics including physical and structural changes in their roots, leaves, and stems. This enables them to absorb significant amounts of water due to their habitat.
Hydrophytes live in a very wet environment, which can cause their roots to be absent or poorly developed. Some hydrophytes lack root caps and root hairs in order to limit water absorption. Floating hydrophytes have root pockets at the end of their roots that act as a balancer and prevent drowning. Other hydrophytes have breathing roots called pneumatophores that grow above the water level to obtain oxygen gas. Hydrophytes also have stems that are spongy, elongated, thick, flexible, and resistant to damage from water movement.
The stems contain numerous aerenchyma cells, which can be found in both the roots and stems. These cells play a role in storing gases required for respiration and create air spaces that help the plant float on water. Additionally, aerenchyma cells enable water diffusion from higher areas of the plant to the roots, allowing them to obtain oxygen without relying on the soil. Additionally, the submerged portions of hydrophytes are coated with mucilage.
Mucilage is responsible for safeguarding the plant body and enabling smooth movements in water. Submerged hydrophytes exhibit either intricately divided leaves or elongated and slender leaves, which aid in minimizing water resistance. On the other hand, floating plants possess broad, tough, and thick leaves to impede the easy diffusion of water into the plants, preventing their drowning. These leaves also offer a significant surface area for photosynthesis and absorption.
The upper epidermis of the leaves contains numerous stomata which capture sunlight for photosynthesis and also facilitate gaseous exchange. In contrast, the lower epidermis lacks stomata due to its contact with water. The chloroplasts necessary for photosynthesis under low light conditions are exclusively found on the upper surface of the leaves because it is exposed to sunlight.
Floating plants have leaves that are coated with a wax layer to prevent wilting and provide protection against chemical and physical damage. The wax also helps to repel rainwater and keep the stomata clear and open. Additionally, hydrophytes have small hairs on their leaf surfaces to ensure stability on the water’s surface and decrease transpiration rate.
Hydrophytes have undergone anatomical adaptations to successfully adapt to aquatic environments. These adaptations include changes in factors such as protecting structure, mechanical tissue, conducting tissue, and aeration. The protecting structure of hydrophytes has been reduced, as the cuticles are absent in their submerged parts. Additionally, the epidermis of the plants no longer serves as a protective organ but rather functions as a photosynthesizing organ. The hypodermis of hydrophytes is poorly developed, which means it is not able to effectively protect the plant from pathogens.
The mechanical tissue in hydrophytes, which provides support and strength, is reduced or poorly developed in the submerged parts due to the plants being supported by water. This absence of mechanical tissue is also seen in the stems and leaf petioles. However, this is not detrimental to the plant’s structure since they are supported by water in their aquatic environment. In some hydrophytes, sclereids serve as a substitute for sclerenchyma cells. Additionally, the conducting or vascular tissue is reduced as the vascular bundles decrease in hydrophytes.
Hydrophytes do not possess xylem because they have the ability to absorb water, dissolved gases, and nutrients directly from their surrounding water. Their internal tissue is well-aerated thanks to parenchyma cells which contain air chambers in the roots, stems, and leaves. These air chambers play a role in photosynthesis, respiration, gas storage, buoyancy increase, and mechanical support.
References
- B.Menezes. (n.d.). Periwinkle General Science Std.5. Mumbai : Jeevandeep Prakashan Pvt. Ltd.
- Cara Lea Council (2002), Plants Adaptation, UNM Biology Undergraduate Labs, The University of Mexico from http://biology.unm.edu/ccouncil/Biology_112/Summaries/Plant_Adaptations.html
- Dr. J. P. Sharma. (n.d.). Comprehensive Biology XII. New Delhi: Laxmi Publications (P) Ltd. Hypodermis. (n.d.). Retrieved on October 10, 2013 from http://www.ecomii.com/science/encyclopedia/hypodermis
- Kevin Byne (2001) Adaptation of Xerophytes and Hydrophytes from http://yunzhenandbio.blogspot.com/2010/06/adaptations-of-xerophytes-and.html#!/2010/06/adaptations-of-xerophytes-and.html
