Final answer:
Aquatic plants on another planet may retain simpler structures for sexual reproduction and lack extensive root systems or a thick cuticle, as they do not need to combat dryness or support themselves against gravity like land plants.
Step-by-step explanation:
On a hypothetical planet where plants evolved similarly to those on Earth but remained aquatic, it's likely they would retain traits that are advantageous in a watery environment. Aquatic plants do not need to invest in structures for water absorption or support against gravity. One trait that might be retained longer than land plants is the lack of extensive root systems, as they are not needed for absorbing water in an aquatic environment. Additionally, these plants might also retain simpler structures for sexual reproduction, where the sperm can still swim to the eggs, due to the abundance of surrounding water.
Moreover, without the evolutionary pressure to combat dryness and UV radiation, these aquatic plants would likely maintain a thinner cuticle or perhaps no cuticle at all, compared to their terrestrial counterparts. Vascular tissues may also be less developed, as water buoyancy reduces the need for structural support to prevent wilting and to transport water and nutrients against the force of gravity - both concerns on land that are not as pressing underwater.