Focusing on the water available condition, the plants are classified as xerophytes, mesophytes, and hydrophytes, respectively. The xeric condition plants grow in extremely dry conditions whereas the mesophyte plants grow in soils with optimum soil water and in the case of hydrophytes, they are aquatic plants that grow in fresh to marine water. 

XEROPHYTES 

The adaptations of xerophytes include, 

• Have modified structures include spines, the presence of hairs, and wax coatings that protect the leaves.  
• The stem and the roots have high level of starch which helps in resistance and stability.
• Have a thick stem which helps to store more water and in photosynthesis.
• Have well-developed epidermis and vascular tissue with well-known xylem and phloem components. 

Based on the adaptation, xerophytes are classified as Ephemeral annuals, Succulents, and Non-succulent perennials. 

Ephemeral Annuals:  

• Found in semi-arid regions.  
• Have a short span of lifetime so these plants are considered as “false xerophytes.” 
• Example, Gramineae (grasses) 

Succulents: 

• Commonly found in semi-arid regions and frequently found in locally dry habitats includes sea beaches and sandy soils etc.
• They save water as gel with the help of water storage tissue cells. The Leaves contain spines that reduce transpiration (Ex: Aloe).
• Stomata is present on the epidermis of the fleshy stems. It remains close during the daytime and open in night-time to absorb CO2. (Ex: Cactus).
• Have a well-developed root system that penetrates to the surface very deep and is more resistant to high temperature and absorbs a maximum of water. (Ex: Cactus).
• Examples include stem succulents is Cactus and leaf succulents include Agave, Aloe. 

Non-succulent perennials:   

• Generally known as “true – xerophytes” can withstand very high temperatures, without water.  
• A thick stem that is covered by brown corky bark. (Ex: Acacia nilotica)  
• The leaves are modified as phyllodes which carry out the photosynthesis (Ex: Acacia melanoxylon)  

• Leaves are thin and can differ based on their living condition (submergence).  
• Roots are poorly developed, and they are very thin and short (Ex: Hydrilla, Nymphaea and Nelumbo) or completely absent.  
• Have a long, slender, and flexible stem (Ex: Hydrilla, Vallisneria) and free-floating forms has a short thick spongy stem (Ex: Eichhornia). In some cases, the stem is well developed in rhizome (Ex: Nelumbo). 
• Cell surface can help in absorption of water and minerals. 
• Have poorly developed vascular system.
• Mechanical tissues are poorly developed or absent. 

Based on their submergence property they are classified into, 3 types namely, Emergent, Floating and Submerged. 

Emergent 

• The plant roots are submerged in water or soil and have a reduced leaf surface.
• They were commonly known as helophytes. (Ex: Typha, Phragmites)  

Floating plants 

• Leaves having a thin, flat structure contains air spaces that gives buoyancy.
• Free- floating leaves contains smooth wax coatings which protects from chemical and physical injuries.
• Stems modified into runners or rhizome. (Ex: Nelumbo, Nymphaea).
• The petioles swollen as bulbous spongy which helps the leaves float on the surface. (Ex: Eichhornia,Nelumbo).
• Epidermal cells contains chloroplasts which helps for photoynthesis and it absorbs water and nutrients from the surface. 

Submerged plants 

• Plants are submerged in water completely. 
• Have poorly developed root system and leaves that circulate the stem. Stomata is completely absent.
• Leaves are thin and possess long internodes and no presence of chloroplasts in epidermal cells. (Ex: Hydrilla, Vallisneria, Najas). 
• The leaf’s upper surface coated with a waxy cuticle.
• The stem is long, slender, and flexible to move with water currents.
• The hydrophytes leaf and stem contains intercellular air spaces called lacunae or aerenchyma. These small air pockets help in exchanging gases such as oxygen and carbon dioxide. 

Learning Outcomes  

• Students learn about adaptations.  
• Students learn about adaptations of xerophytes.
• Students learn about adaptations of hydrophytes.