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Why do aquatic plants generally have less-developed xylem than land plants?

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temperate softwoods and hardwoods selected to show variations — wood Temperate softwoods (left column) and hardwoods (right column), selected to highlight natural variations in colour and figure: (A) Douglas fir, (B) sugar pine, (C) redwood, (D) white oak, (E) American sycamore, and (F) black cherry. Each image shows (from left to right) transverse, radial, and tangential surfaces. Click on an individual image for an enlarged view.

Aquatic plants generally have less-developed xylem than land plants because they do not require the same specialized structures for structural support or long-distance water transport.12

On land, plants must overcome gravity and the drying effects of the atmosphere, necessitating a rigid, self-supporting structure provided by xylem to remain upright and transport water from the soil to the leaves. In contrast, aquatic plants are supported by the buoyancy of the surrounding water, which eliminates the need for a rigid stem. Furthermore, because these plants are submerged, they do not face the same challenges regarding water scarcity as terrestrial plants, meaning they have little need for extensive water-conducting tissues. Consequently, plants that have evolved to return to an aquatic habitat often exhibit a reduction in xylem, as they no longer require the complex vascular systems essential for life on land.12