Anatomy and Ultrastructure of Wolffia columbiana and Wolffia borealis, Two Nonvascular Aquatic Angiosperms

Abstract

Light and electron microscope studies of Wolffia columbiana Karsten and Wolffia borealis (Engelmann ex Hegelmaier) Landolt & O. Wildi vegetative stages were undertaken. Asexual reproduction in both species involves the budding of daughter fronds from a meristematic region in the reproductive pouch of a mother frond. A single Wolffia plant (mother plus daughter plus granddaughter fronds) is composed of six to seven individuals at various developmental stages. The mesophyll of W. columbiana (especially in older fronds) is composed of thin-walled cells with few organelles; chloroplasts are concentrated instead in the epidermis and have a normal ultrastructure consisting of both appressed and unappressed thylakoids. Anticlinal cell walls of adjacent W. columbiana epidermal cells have elaborate cell wall ingrowths characteristic of transfer cells. Wolffia borealis mesophyll cells vary in size from small cells toward the dorsum to large cells toward the ventrum. Chloroplasts are concentrated in the small dorsal mesophyll cells and lack significant thylakoid appression. Transfer cells are absent in W. borealis. X-ray microanalysis indicated high levels of calcium on the W. borealis surface, with even higher concentrations in the punctae cells. Calcium oxalate was not detected. The W. columbiana transfer cells and the agranal W. borealis chloroplasts are features not previously reported in the Lemnaceae.