Water enters the root and is transported up to the leaves through specialized plant cells known as xylem pronounces zigh-lem. Land plants must guard against drying out desiccation and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf cuticle, but it can enter the leaf through an opening the stoma; plural = stomata; Greek for hole flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal water is also lost. Cottonwood trees, for example, will lose 100 gallons of water per hour during hot desert days. Carbon dioxide enters single-celled and aquatic autotrophs through no specialized structures.