| MadSci Network: Botany |
This is an interesting question. Although leaves are the main photosynthetic organs of many plants, stems also carry out photosynthesis in many species. Elodea stems and leaves both photosynthesize. The Elodea photosynthesis experiment you describe is artificial in that the shoots had their roots removed and the shoots are upside down. Submerged stems of aquatic plants or hydrophytes typically contain large intercellular air spaces. Plant tissue with large intercellular air spaces is termed aerenchyma and also occurs in the leaf mesophyll of terrestrial plants. These air spaces provide a submerged or partly submerged plant with a pathway to channel oxygen to the roots as well as a channel for carbon dioxide from the roots to reach the shoots. During rapid photosynthesis, gas pressure can build up in the intercellular air spaces of an aquatic plant. The oxygen gas escaping from the cut end of the cut, inverted Elodea shoots follows the air channels in the aerenchyma. Less gas escapes from the leaves because those cells are more tightly packed and provide greater resistance to gas flow than aerenchyma. Also, gas moves much slower in water than in air spaces. Elodea can excrete some oxygen bubbles from their leaves during photosynthesis. Submerged leaf disks cut from terrestrial plant leaves will produce oxygen bubbles (see leaf disk photosynthesis webpage below). Just be sure oxygen bubbles are not forming because your light source is heating the solution and causing dissolved gases to form gas bubbles on surfaces. An incandescent light bulb produces a lot of heat, and gas solubility in water decreases as the water temperature increases. References Leaf Disk Photosynthesis Demo Aerenchyma Aerenchyma formation Aerenchyma in Woody Plants Aquatic Plants
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