Angiosperms do not require external water for fertilization. Unlike mosses and ferns, which need a film of water for sperm to swim through, flowering plants deliver sperm directly to the egg cell through a structure called a pollen tube. This internal delivery system is one of the key adaptations that allowed flowering plants to thrive in dry environments across the globe.
How Angiosperms Fertilize Without Water
In mosses, sperm physically swim through water droplets to reach a neighboring plant’s egg. Ferns work similarly: even tree ferns depend on their ground-level gametophyte stage having access to moisture so sperm can travel. Both groups are tied to wet habitats for reproduction.
Angiosperms bypassed this limitation entirely. Instead of releasing swimming sperm into the environment, a flowering plant packages its sperm inside pollen grains. Wind, insects, birds, or other animals carry pollen from one flower to another. Once a pollen grain lands on the sticky surface of a flower’s stigma, it germinates and grows a narrow tube down through the style toward the ovule. The two sperm cells travel inside this tube, never exposed to the outside environment. In corn, pollen tubes elongate at 2 to 12 millimeters per hour, fast enough to traverse silk lengths of 25 centimeters.
This method of delivering sperm through a tube rather than through environmental water is called siphonogamy. It replaced the ancestral system (zooidogamy) in which plants produced motile, flagellated sperm that needed liquid to move. Basal land plants like mosses and ferns still use zooidogamy. Most seed plants, including all angiosperms, use siphonogamy with nonmotile sperm.
Where the Water Actually Comes From
While angiosperms don’t need rain, puddles, or dew for fertilization, the process isn’t completely water-free at the cellular level. Pollen grains arrive at the stigma in a partially dehydrated state and need moisture to germinate and begin tube growth. The plant itself provides this moisture internally.
Flowering plants handle this in two ways depending on species. Plants with “wet” stigmas produce a fluid from stigma cells before pollination even occurs, creating a ready environment for any pollen that arrives. Plants with “dry” stigmas supply water directly from specialized surface cells called papillae, and this transfer is selective, meaning the stigma can preferentially hydrate compatible pollen while rejecting foreign species. Either way, the water comes from the plant’s own tissues, not from the external environment.
Double Fertilization: A Unique Angiosperm Feature
Once the pollen tube reaches the ovule, something happens in angiosperms that occurs in no other plant group. Two sperm cells are released, and both participate in fertilization. One sperm fuses with the egg cell, producing the embryo that will grow into a new plant. The second sperm fuses with another cell called the central cell, producing a nutrient-rich tissue called endosperm that feeds the developing embryo inside the seed.
This double fertilization is nearly universal among angiosperms. A handful of exceptions exist: the riverweed family (Podostemaceae) and some orchids undergo only single fertilization, where the second sperm cell either never forms or disintegrates. But in all other studied flowering plant families, both fusion events occur.
The Exception: Aquatic Angiosperms
A small group of flowering plants reversed course evolutionarily and actually do use water to transport pollen. About 18 genera of fully submerged aquatic angiosperms practice true hydrophily, meaning water itself carries pollen to the female flower.
This takes two forms. In ephydrophily, pollination happens at the water’s surface. Vallisneria, a common aquarium plant, releases tiny male flowers that float on the surface and drift into contact with female flowers. In hyphydrophily, everything happens underwater. Zostera (eelgrass) releases thread-like pollen that drifts through seawater to reach submerged female flowers. Some genera in the pondweed family (Potamogetonaceae) even use water to facilitate self-pollination.
These aquatic species are secondary adaptations, though. Their ancestors were land plants that pollinated through wind or animals. They re-evolved a dependence on water for pollen transport after moving into aquatic habitats. Even in these species, the actual fusion of sperm and egg still happens inside the ovule via a pollen tube, not through free-swimming sperm.
How This Compares to Other Plant Groups
The distinction becomes clearest when you line up the major plant groups. Mosses and liverworts are completely dependent on external water: sperm are released into moisture on the plant surface and swim, guided by chemical signals, to the egg. Ferns and their relatives work the same way. Even large tree ferns need ground-level water during their sexual reproduction phase.
Gymnosperms (conifers, ginkgoes, cycads) mostly use pollen tubes like angiosperms, but a few living fossils straddle the line. Cycads grow pollen tubes but still produce flagellated, motile sperm that swim a short distance inside the ovule once released. They represent a molecular intermediate between the ancient water-dependent system and the fully tube-based system of flowering plants.
Angiosperms completed the transition. Their sperm are nonmotile, carried passively through the pollen tube, and never encounter the outside environment. This independence from environmental water for reproduction is one reason flowering plants dominate nearly every terrestrial ecosystem on Earth, from deserts to mountaintops.