Plumbing made of dead cells let plants grow tall and conquer land
A tree lifts water tens of metres without a pump, spending almost no energy, through pipes built from cells that are already dead. That internal plumbing defines the vascular plants, about 300,000 known species from ferns and horsetails to pines and roses, and it is what allowed them to outgrow mosses.
Botanists pick out vascular plants by three traits. They have conducting tissues, xylem for water and minerals and phloem for sugars, usually bundled side by side. Their dominant life stage is the diploid sporophyte, carrying two chromosome sets, whereas in mosses the haploid gametophyte dominates. And they have true roots, stems and leaves, though some lineages later lost one. The group includes clubmosses, horsetails, ferns, conifers and other gymnosperms, and flowering plants. The earliest known one, Cooksonia, appeared in the mid-Silurian.
Xylem consists of hollow, hard-walled dead cells lined up into tubes, called vessels in flowering plants and tracheids in the rest, their walls stiffened by lignin. Phloem is alive: sieve-tube cells joined by perforated plates carry sucrose from leaves, the source, to growing shoots and roots, the sinks. The sieve tubes lack nuclei and ribosomes, relying on neighbouring companion cells to keep them running. Secondary xylem is simply wood, the raw material of the timber industry.
Water climbs mainly by transpiration. As moisture evaporates from leaf pores called stomata, surface tension in the leaf cells tugs on the water column below, and hydrogen bonds link each molecule to the next, so the whole chain is drawn upward from the roots. The process can be entirely passive, and it carries dissolved mineral salts along with it. At night, with stomata shut, pressure can build, and plants push excess water out through pores called hydathodes.
Conducting tissue let these plants reach sizes impossible for non-vascular ones. One idea for why the diploid stage came to dominate is that taller, more elaborate spore stalks could scatter spores farther. Older books called vascular plants higher plants, a label now considered unscientific because it echoes the discredited ladder-of-nature view of evolution.
Source: Vascular plant
