Leaving the Water
Land offered abundant light, plentiful carbon dioxide and, at first, no competitors or predators. It also posed problems that water had solved automatically.
| Problem | Solved in water by | Solution on land |
|---|---|---|
| Desiccation | Being surrounded by water | Waterproof cuticle, skin, exoskeleton |
| Support | Buoyancy | Lignin, skeletons, exoskeletons |
| Gas exchange | Dissolved gases | Stomata, lungs, tracheae |
| Reproduction | Water carries gametes | Pollen, seeds, internal fertilisation |
| Transport | Diffusion in a small body | Xylem and phloem, circulatory systems |
Plants solved these in a sequence still visible in the tree. Early bryophytes had no vascular tissue and stayed small and damp. The cormus — true roots, stem and leaves — arrived with vascular tissue: xylem carrying water upward and phloem carrying sugars, both stiffened by lignin, which is what allowed height. The seed freed reproduction from standing water, and the flower recruited animals as pollinators, which is far more targeted than releasing pollen to the wind.
Water rises in xylem without a pump. Evaporation from the leaves creates tension, and cohesion between water molecules transmits that tension down an unbroken column, the cohesion-tension mechanism, pulling from the top rather than pushing from below.
Fungi took a different route: a mycelium of fine hyphae with an enormous absorptive surface, feeding by external digestion, secreting enzymes and absorbing the products. Their partnership with plant roots, the mycorrhiza, is ancient and may have been what made plant colonisation possible at all.
Animals needed waterproofing, internal fertilisation or shelled eggs, and internal gas exchange. Arthropods managed it first with an exoskeleton that solved support and desiccation together. Vertebrates needed the amniotic egg before they could breed away from water entirely.
Common pitfall: describing water as being "pushed" up a tree by the roots. Root pressure exists but is far too weak for a tall tree. The column is pulled from the top by evaporation, held together by the cohesion of water, a tension so strong that a severed trunk draws air in rather than leaking water out.