Gaseous exchange takes place by diffusion. The structure of the leaf is adapted for gaseous exchange by having intercellular spaces that are filled. These are many and large in the spongy mesophyll.
When stomata are open, carbon(IV)oxide from the atmosphere diffuses into the substomatal air chambers. From here, it moves into the intercellular space in the spongy mesophyll layer.
The CO2 goes into solution when it comes into contact with the cell surface and diffuses into the cytoplasm.
A concentration gradient is maintained between the cytoplasm of the cells and the intercellular spaces. CO2 therefore continues to diffuse into the cells.
The oxygen produced during photosynthesis moves out of the cells and into the intercellular spaces.
From here it moves to the substomatal air chambers and eventually diffuses out of the leaf through the stomata. At night oxygen enters the cells while CO2moves out.
Gaseous exchange in the leaves of aquatic (floating) plants
Aquatic plants such as water lily have stomata only on the upper leaf surface. The intercellular spaces in the leaf mesophyll are large. Gaseous exchange occurs by diffusion just as in terrestrial plants.
Observation of internal structure of leaves of aquatic plants
Transverse section of leaves of an aquatic plant such as Nymphaea differs from that of terrestrial plant.
The following are some of the features that can be observed in the leave of an aquatic plant;
- Absence of cuticle
- Palisade mesophyll cells are very close to each other ie.compact.
- Air spaces (aerenchyma) in spongy mesophyll are very large.
- Sclereids (stone cells) are scattered in leaf surface and project into air spaces.
- They strengthen the leaf making it firm and assist it to float.
Gaseous Exchange Through Stems Terrestrial Plants
Stems of woody plants have narrow openings or slits at intervals called lenticels. They are surrounded by loosely arranged cells where the bark is broken.
They have many large air intercellular spaces through which gaseous exchange occurs. Oxygen enters the cells by diffusion while carbon (IV) oxide leaves. Unlike the rest of the bark, lenticels are permeable to gases and water.