why do underwater plants have long thin leaves

They have cellular adaptations that make it possible for them to get the chemicals they need underwater. Besides trace nutrients that they obtain from soil around their roots,
the main needs of a plant is water (H2O, which they are surrounded by) and carbon dioxide (CO2, which readily dissolves in water). The biggest issue for underwater plants is light, so the deeper and less clear the water, the more challenging it is for plant life to exist.

We will discuss about the plants living in water. Plants growing in water are called aquatic plants. There are three types of aquatic plants. They are: (i) Floating plants, (ii) Fixed plants, (iii) Under water plants. There are some plants which float on water.

Some plants like duckweed, green-alge, wolfia, water-hyacinth and pistia are some of the floating plants that float freely on top of the water. They are called floating plants. They have spongy bodies. Like a sponge there are lots of empty spaces throughout their body and are filled with air.

This makes the plants light enough to float. Some plants like water-lily and lotus have roots that fix the plants in the mud at the bottom of the pond. They are known as fixed aquatic plants. Roots of such plants are fixed in the soil at the bottom of a pond. They have plate-like leaves that float over the surface of water. The stomata in the leaves are on the upper side.

The stems are hollow and very light. This helps the leaves to float. The stems are very flexible. They bend with the flow of water so they do not get damaged by strong current. Some plants like pondweed, tape-grass, hydrilla, etc. are some common plants which live and grow under water completely.

They are called underwater or submerged plants. Their roots fix them in the muddy soil. These plants have narrow, thin leaves without pores. These plants move along with the current. Such plants have very delicate shoots. They breathe through their body surface. The stems are flexible and have air spaces. We can grow such plants in aquarium.