USES. Many species have pungent, and even _poisonous properties_ (_e.g. Dieffenbachia_, _Lagenandra_, _Arum_), which are easily removed by boiling or roasting; the _rhizomes_ of many species of _Caladium_, _Colocasia_ (_C. antiquorum_, _esculenta_, etc.), are very rich in starch, and in the Tropics form an important source of food. An uncommon occurrence in the order is the highly aromatic rhizome of _Acorus calamus_; this contains calamus-oil and acorin which are used in perfumery. Many are ornamental plants, _e.g. Zantedeschia æthiopica_ (South Africa), generally known as “Calla,” and _Monstera deliciosa_; many other species are grown in greenhouses. Order 6. =Lemnaceæ (Duck-weeds).= These are the most reduced form of the Spadicifloræ. They are very small, free-swimming water-plants. The vegetative system resembles a small, leaf-like body (Fig. 303 _f-f_), from which roots hang downwards; this branches by producing a new, similar leaf-like body, which springs from a pocket-like hollow (indicated by a dotted line in the figure) on each side of the older one, at its base (or only on one side). ~The branching is thus dichasial or helicoid (Fig. 303 _A_, where _f, f′, f″, f″′_ indicate shoots of 1st, 2nd, 3rd, 4th generations respectively). The leaf-like bodies are, according to Hegelmaier, leaf-like stems, and thus _Lemna_ has no other leaves than the spathe and the sporophylls; according to the investigations of Engler they are stems whose upper portion (above the “pocket”) is a leaf, which is not sharply separated from the underlying stem-portion. The inflorescence is a very much reduced Araceous-spadix, consisting in _Lemna_ of 1 or 2 stamens of unequal length (1-stamened ♂-flowers), 1 unilocular carpel (♀-flower), and 1 thin spathe (_B_). [The same is found in _Spirodela polyrrhiza_, etc., whose daughter-shoots begin in addition with 1 basal-leaf. _Wolffia arrhiza_, etc., have no roots, no spathe, and only 1 ♂-flower in the inflorescence (Engler).]--On the germination of the seed a portion of the testa is thrown off as a lid, so that an exit is opened for the radicle.--19 species. In stagnant fresh water, both Temp. and Tropical.--In Europe the species are _Lemna minor, trisulca, gibba; Spirodela polyrrhiza_, and _Wolffia arrhiza_, the smallest Flowering-plant.~ [Illustration: FIG. 303.--_Lemna_: _A_ vegetative system; _B_ portion of a plant with flowers; one stamen and tip of the carpel project; the remaining portions being indicated by the dotted line.] Family 4. =Enantioblastæ.=