The most abundant tissue seen in Hydrilla and Eichhornia plants is __________.
A. Aerenchyma
B. Collenchyma
C. Sclerenchyma
D. Phloem

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Hint: Hydrilla and Eichhornia are aquatic plants that float in the water. These plants have spongy tissue all over their bodies that provides them buoyancy. Also, it provides air channels in the stems, roots, and leaves of some plants.

Complete answer: Aquatic plants or hydrophytes are those plants that are found either submerged or floating on the water bodies. Both Hydrilla and Eichhornia are aquatic plants that stay submerged or floating in the water respectively. Their stems and other parts float freely. Hydrilla is found in the warm and cool waters of Asia, Africa, and Australia. Its stems are of length 1 to 2 meter. The leaves are arranged in whorls around the stem. Leaves have small spines along their margins. Eichhornia which is called water hyacinth is a floating type aquatic plant. It is a flowering aquatic plant with light purple colored flowers. Both of these plants have a spongy tissue that forms air channels in the stems, leaves, and roots of the plant that provides buoyancy in them. This tissue is called aerenchyma. As the name suggests aerenchyma allows for gaseous exchange between shoot and root. The tissue provides channels of air-filled cavities. This provides a low-resistance internal pathway that allows for the exchange of gases like oxygen and ethylene. These gases are exchanged between the plant above the water and the submerged tissues. Thus, aerenchyma is a widespread tissue in aquatic plants like Hydrilla and Eichhornia.
So, option A is correct.

Note: In the low oxygen conditions of flooded soils, the microorganisms consume oxygen at a faster rate. Most wetlands have hypoxic soils (soils deficient in oxygen). Water lilies are an example of plants that are found in such conditions. They are wetland plants and they also contain an abundance of aerenchyma.