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Key to Azolla

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1 Fronds regularly 2 (-3)-pinnate, generally equilaterally triangular in outline; megasporangia with 9 floats on exospore; microsporangial massulae with few simple or irregularly branched silklike hairs; lateral branches as many as fronds; [section Rhizosperma]
1 Fronds more irregularly compound (see below), lobulate and irregular in outline; megasporangia with 3 floats on exospore; microsporangial massulae with several anchorlike hairs; lateral branches fewer than fronds; [section Azolla].
..2 Largest hairs on the upper leaf surfaces 1-celled; plants usually (0.7-) 2-6 cm long, elongate and pinnate in its main axis (pseudodichotomously branched only peripherally); megaspores with raised hexagonal markings/angular bumps
..2 Largest hairs on the upper leaf surfaces 2-3-celled; plants usually 0.5-1.5 cm long, either primarily pinnate or primarily pseudodichotomous (see below); megaspores lacking raised hexagonal markings/angular bumps, but sometimes pitted.
....3 Plants usually 0.5-1 cm long, the stems pseudodichotomously branched nearly throughout; megaspores (which are rarely present) not pitted, densely and uniformly covered with tangled filaments (use 100× magnification); leaves not or only slightly imbricate, the upper lobes usually 0.5-0.6 mm long
....3 Plants usually 1-1.5 cm long, the stems more or less pinnately branched (with a central axis), pseudodichotomously branched only at outer periphery or frond; megaspores (which are rarely present) pitted, sparsely covered with a few long filaments (use 100× magnification); leaves slightly to greatly imbricate, the upper lobes usually > 0.7 mm long
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