Why Christmas trees are not extinct

Image: The electron microscope images show bird's-eye-views of the valves that allow water to move upward through conifer trees (upper left) and angiosperms or flowering trees (upper right), while the graphics show cross-sections of water flow through both kinds of valves. Conifers -- including Christmas trees -- have valves with a central "torus" through which water cannot pass, surrounded by a porous "margo" through which water flows easily. Flowering trees have homogenous porous valves through which water flows less efficiently. Credit: Springer Science and Business Media, G.L. Comstock, W.A. Cote and E. Wheeler.