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Most fronds collected from Liz Pelonzi Pool, NH USA in early August 2010 had numerous red spots, especially in the outer epidermal cells, referred to as idioblasts (Hegelmaier 1868), pigment cells (Hillman 1961), and punctae cells (White and Wise 1998). The pigment may be anthocyanin, perhaps initiated by high UVB intensity in mid-summer. Spot size is relatively uniform at ~ 8 10 m. Experiments with Spirodella oligorhiza and Wolffia punctata demonstrated convincingly the conversion of colorless flavans in idioblasts to red-brown compounds (Witztum 1974) that correspond to the currently observed bright red spots.

References:

Hegelmaier, F. 1868. Die Lemnaceen eine monographische Untersuchung. Wilhelm Engelmann, Leipzig.

Hillman, W.S. 1961. The Lemnaceae or duckweeds a review of the descriptive and experimental literature. Bot. Rev. 27:221-287.

White, S.L., and R.R. Wise 1998. Anatomy and ultrastructure of Wolffia Columbiana and Wolffia borealis, two nonvascular aquatic angiosperms. Int. J. Plant Sci. 159:297-304.

Witztum, A. 1974. Ultraviolet irradiation and pigment cell idioblasts in Spirodela oligorhiza (Lemnaceae). Amer. J. Bot. 61:713-716.