Stomata are bordered by guard cells, which can open or close the stomata. In order to maintain homeostasis, the guard cells the stomata when water levels in the plant's cells become too low, and they the stomata when carbon dioxide levels in the cells become too low.

Which of the following best describes the expensive tissue hypothesis? brains require more energy, so the gut had to be reduced, and larger brains and tool use led to higher quality diets. increasing body size means that homo neanderthalensis had to include more fat in its diet. brains require more energy, so the gut had to be reduced as brains got bigger. brains require more fat, so the gastrointestinal viscera (gut) had to expand, and hands were needed to acquire more food.