Most gymnosperms lack vessels in their xylem (unlike flowering plants which have both vessels and tracheids), except for the gnetophytes, which have vessels. ... This means that the movement of pollen (male gamete) to ovule (female gamete) in seed plants relies on airborne transport, not water transport.

Why gymnosperms lack vessels?

Gymnosperms are a group of plants that include conifers, cycads, and ginkgos. Unlike angiosperms (flowering plants), gymnosperms lack vessels in their xylem tissue. The absence of vessels in gymnosperms is due to several factors related to their evolutionary history and ecological adaptations.

Evolutionary history:
1. Primitive morphology: Gymnosperms are considered to be more primitive than angiosperms. They evolved earlier in plant evolution and have retained some characteristics of their ancestors, including the absence of vessels.
2. Evolution of tracheids: Gymnosperms have tracheids, which are elongated cells that serve as the main conducting cells in their xylem tissue. Tracheids are structurally simpler than vessels and are thought to have evolved before vessels. Therefore, gymnosperms did not acquire the ability to form vessels during their evolutionary history.

Ecological adaptations:
1. Drought tolerance: Gymnosperms are often found in environments with limited water availability, such as forests, deserts, and high altitudes. The absence of vessels in their xylem tissue helps them withstand drought conditions. Tracheids have smaller diameters and more narrow pits compared to vessels, which reduces the risk of embolism (blockage of water flow) during water stress.
2. Resistance to freezing: Gymnosperms also inhabit cold climates where freezing temperatures occur. The absence of vessels in their xylem tissue helps prevent ice formation and subsequent damage. Vessels, with their larger diameters, are more susceptible to ice formation, whereas tracheids are better adapted to withstand freezing conditions.
3. Long-distance water transport: Gymnosperms often grow to large sizes and can transport water over long distances. Tracheids, with their narrower diameter and interconnectedness, provide effective water transport throughout the plant.

In conclusion, gymnosperms lack vessels in their xylem tissue due to their evolutionary history and ecological adaptations. The absence of vessels contributes to their drought tolerance, resistance to freezing, and efficient long-distance water transport.