Because hydrophtes are supported by water rather than by roots and stem structures, most hydrophtes as a result have roots that are small and feathery. These are designed to take in oxygen from the water, and since the plant is fully submersed in water at all times, there is less need for a long and thick root structure. One plant that has a feathery root is the Salvinia Molesta, which is a free-floating aquatic fern.

Often found on floating plants, many hydrophytes have air sacs that help the plant float on the surface of the water. The water chestnut is one example, which includes a beautiful cluster of green leaves with white flowers, attached to a fine root system. It is important to note that some aquatic plants will float slightly submerged in the water, such as the buttercup. Others, such as water lilies, will float up on top of the surface since their leaves distribute the weight across the surface of the water. 

Hydrilla is a genus of aquatic plant, usually treated as containing just one species, Hydrilla verticillata, though some botanists divide it into several species. Opuntia, touch me not and Calotropis lives on land. Thus, option B is correct and other options are wrong.

About 72% of the land on the Earth is a part of the oceans. This water of the oceans, rivers, lakes and ponds contribute to the aquatic ecosystem. So, about 72% of the Earth's surface is covered under the aquatic form of ecosystems. There are two types of aquatic ecosystem, freshwater and marine ecosystem. 

In a freshwater aquatic ecosystem like a pond, the organisms in the food chain include algae, small animals, insects and their larvae, small fish, big fish and a fish-eating bird or animal.

Aquatic plants are plants that have adapted to living in aquatic environments (saltwater or freshwater). They are also referred to as hydrophytes or macrophytes. These plants require special adaptations for living submerged in water, or at the water's surface. The most common adaptation is aerenchyma, but floating leaves and finely dissected leaves are also common. Aquatic plants can only grow in water or in soil that is permanently saturated with water. They are therefore, a common component of wetlands. 

Succulent plants stores water in their stems, fleshy leaves, such as the aloe.

Victoria amazonica is commonly known as Giant Water Lily. They have the lagest floating leaves. They have large floating leaves, up to 3 meter in diameter. The leaves float on the surface of water on a submerged stalk.

In submerged hydrophytes, the entire plant body is involved in the absorption of water and these plants not experiences much stress or pressure internally. So in these plants, xylem and mechanical tissues are poorly developed.

So, the correct option is ‘Poor development of xylem and mechanical tissues’.

A.Submerged hydrophytes – plants grow completely in the water, stomata are absent.

B.Hygrophytes – plants grow in wet conditions, stomata are present.

C.Mesophytes – plants grow in average climatic conditions. Stomata are present.

D.Xerophytes – plants grow in dry or arid conditions, sunken stomata are present.

So, the correct option is ‘submerged hydrophytes’.

In hydrophytes or aquatic plants, large intercellular air spaces are developed extensively in the parenchyma so as to form a connected system throughout the plant. Such a modified parenchymatous tissue is called aerenchyma. It provides buoyancy to the plants and helps in gaseous exchange also. Thus the correct answer is option B.

A. Nymphaea- Reserve food material in perisperm (Persistant nucellus). Perisperm occurs in the seeds of black pepper, coffee, castor, cardamom, Nymphaea. It is diploid.

B. Coconut- Reserve food material in endosperm.

C. Onion- Reserve food material in endosperm

D. Litchi- Reserve food material in cotyledons.

So, the correct answer is 'Nymphaea'.

Aerenchyma is a spongy tissue that forms spaces or air channels in the leaves, stems and roots of submerged and aquatic plants. The floating leaves are there due to this feature of aerenchyma only. The aquatic plants or hydrophytes do not have submerged leaves or a well developed phloem, So the correct answer is 'Aerenchyma.'

Hydrophytes are aquatic plants that live in water and are evolutionarily suited to thrive in aquatic environments. Hydrilla, also called waterthyme is a submerged hydrophyte, which is a troublesome aquatic weed at many places.

The plants which can live only in water are called hydrophytes. Among these hydrophytes, there are different sizes and classification based on them. The large sized hydrophytes are called macrophytes. As macro suggests large size.

The body of the fish is supported by endoskeleton made up of bones. Fish possess a streamlined body which helps them to swim in water. Fish swim with the help of their fins. Streamlined body helps in orientation of the fish movement in a particular path.

Generally, phosphorus is the limiting nutrient in freshwater aquatic systems. That is, if all phosphorus is used, plant growth will cease, no matter how much nitrogen is available. Phosphorus functions as the growth-limiting factor because it is usually present in very low concentrations. The natural scarcity of phosphorus can be explained by its attraction to organic matter and soil particles. Any unattached or free phosphorus is quickly removed from the aquatic system by algae and larger aquatic plants. Excessive concentrations of phosphoruscan quickly cause extensive growth of aquatic plants and algal blooms. Several detrimental consequences may result. Excessive algae and plant growth can lead to depletion of the oxygen, that is dissolved in the water. 

Root cap is present at the tip of the root and it contains meristematic cells which divide and increase the length of the root.
Mesophytes are terrestrial plants. Therefore, root cap is essential in mesophytic plants.
Epiphytes are plant which grows upon another plant or any object. They require physical support for the growth. Such plants also possess roots which anchor the plant to another plant or object.
Xerophytes are terrestrial plants which have adapted to survive in an environment with little water. Hence, roots are necessary for xerophytes. 
Hydrophytes grow in water. Hydrophytes have plenty of water hence, the root system takes secondary importance. Therefore, root cap is absent in hydrophytes. 
Therefore, the correct answer is 'Hydrophyte'.

A continuous system of air passages from leaves to roots is found in emergent hydrophytes because these are the type of plants that keep themselves up from the top of water and these air passages provide buoyancy to it. submerged plants are sinked in water, floating plants are there on the surface of water. suspended hydrophyte are the plants that are just below the surface of water. So the correct answer is ' Emergent hydrophytes '. 

The petiole is the stalk that attaches the leaf blade to the stem. Water hyacinth leaves in natural populations vary from being long and thin‐petioled to being short with inflated petioles. water hyacinth is a floating hydrophyte that has an inflated petiole which helps in keeping it on top of the water level thus allowing it not to sink. So the correct answer is ' Eichhornia or water hyacinth. '

The statement- the set of methods used in classification based on evolutionary relationship is called phenetics is wrong.