Dicotyledonous plants extract the gases they require through their leaves. They enter the plant through the stomata which are mainly located on the underside of the leaf. They then flow into the intercellular space from which they can diffuse into the cells.

As with an insect’s spiracles, the stomatal opening can be open or closed. This is achieved via guard cell pairs which prevent the plant from losing to much water through transpiration. If the plant’s cells have enough water, the water enters the guard cells by osmosis and they become turgid. Their inner walls are quite inelastic and so, as they become turgid, the pairs bend away from each other, causing the pore to open. However, if there’s not enough water, the guard cells lose their turgidity and shrink in size, thereby closing the pore.

It’s important for the stomata to stay open as much as possible during the day so that carbon dioxide can be used for photosynthesis, which tends to be faster due to the daylight. Respiration, on the other hand, which requires oxygen can occur in the day or night.

Root Pressure

Root pressure also plays an important part in water transportation by increasing the hydrostatic pressure at the bottom of the plant. Cells which surround the xylem vessels are able to pump solutes in the xylem using active transport. This causes a decrease in the water potential of the xylem’s solution thereby causing the root cells to absorb more water. As more water enters the roots so the hydrostatic pressure is increased.