canadian pondweed does not have stomata. How then does carbon dioxide enter the leaves of the plant so that photosynthesis can take place?

Completely submerged water plants like seagrasses and pondweed (Elodea canadensis) lack stomata so they derive nearly all of their photosynthetic carbon through their roots.

Because the other life in the pond will release CO2 into the water where it will dissolve and deposit on the sea floor, CO2 is taken up by the pondweed through its roots.

Photosynthetic organs of most higher plants normally have access to atmospheric CO2 through stomatal pores which also serve as variable valves to control the loss of H2O vapour which accompanies CO2 uptake.

The acquisition of stomata is commonly thought to have been a crucial development permitting ‘conquest’ of land and direct access of plants to atmospheric CO2. Only in desert stem succulents during drought do stomata remain so tightly closed in the light that the photosynthetic tissues are dependent on internal CO2 generated through the photosynthetic pathway known as crassulacean acid metabolism

Functional stomata are absent in submerged aquatic plants and in non-vascular land plants (for example, mosses) which are normally covered by a water film. Although it is now clearly established that some aquatic plants assimilate large amounts of CO2 from the sediment via roots, terrestrial plants are thought to assimilate only insignificant amounts of CO2 via this path. - Nature.com

See Sciencing.com's Photosynthesis in Aquatic Plants to learn more.

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