Morphologically, duckweeds are poorly differentiated hydrophytes that only produce connected fronds with a single or multiple root system on each frond. Such morphological reduction in small, free-floating plants of Lemna and Spirodela has led to the speculation of their simple anatomy. In the current study, complex structural organization, in respect to the cellular differentiation, is revealed within each organ of the Lemna and Spirodela plant body. In particular, laterally connected translucent stalk cells, commonly with polymorphic mitochondria, fibrillar supporting structures and unevenly thickened walls are the most noticeable features. A distinct boundary layer filled with fibrillar materials at the root-root cap junction, chloroplasts having grana with 3-9 stacked thylakoids and starch grains distributed throughout the plant are other interesting structural attributes drawing attention in the study. In summary, characteristics such as an entirely chlorenchymatous plant body, poorly developed vescular tissue, well-established plasmodesmatal connection, rapid vegetative reproduction, offspring protection, aerenchyma formation, effective abscission in the connective stalk and a well-organized root proper having a prominent root cap are clearly demonstrated in these greatly reduced species. Such reduction and differentiation of the plant body effectively contribute to the better adaptation of smaller plants to superficial aquatic environments, while enabling rapid growth.

Key words: adaptation, effective organization, Lemna, Spirodela, structural reduction