The vegetative morphology of Pistia and the Lemnaceæ

The nature of the duckweed “frond” has presented a baffling problem to botanists since the early days of vegetable morphology. A detailed history of the views that have been held on the question need scarcely be attempted here, since the extreme reduction of the Lemnaceæ has offered scope for wild surmises of little scientific value. The principal theories which have been put forward may, however, be summarised as follows :— The “fronds” of the Lemnaceæ have been regarded as :— (1) Entirely axial (Hegelmaier, F. (1868)). (2) Entirely foliar (Goebel, K. (1891-3)). (3) Foliar in the distal region and axial in the proximal region (Horen, F. van (1869), Engler, A. (1877), and Velenovský, J. (1907)). The objections to (1) and (2) are obvious: these views can only be maintained if—in the first case—certain essentially foliar qualities be attributed to stem organs, and—in the second case—if leaves be assumed to possess some of the distinctive properties of stems. It is not inconceivable that such assumptions might find justification, but they should only be used as a last resort, in the case of no adequate explanation on normal lines being forthcoming.