Acoraceae

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Acoraceae

Description

Perennial, glabrous, aromatic herbs, growing in marshes or as emergent aquatics; Leaves distichous, bases overlapping, unifacial, ensiform, not differentiated into petiole and blade; Inflorescence solitary, terminal, borne laterally on leaf-like scape (usually interpreted as peduncle and spathe; Flowers bisexual, perigoniate, densely arranged, bractless, trimerous;

Distribution

Asia-Tropical:, New Guineapresent Europe: North America: Temperate, subtropical and tropical Asia and North America: present
Temperate, subtropical and tropical Asia and North America, introduced and naturalized in Europe and partly in North America, also at least in New Guinea. The genus Acorus is mainly found in the northern hemisphere from the temperate zone to the tropics of Asia and in Malesia. Acorus calamus is distributed in the whole range of the genus, but A. gramineus only in East and Southeast Asia.

Ecology

The genus Acorus contains herbaceous helophytes in , , . Acorus gramineus grows also in or . Both species form large dense stands due to their extensively branched rhizomatous stems. This genus is found . It grows .

Pollination

The flowers of Acorus are proterogynous. The pollination was unknown until now (26/27 February 2011) when Luo Shixiao observed at night Contarinia spec. (Cecidomyiidae), a gall midge, as pollinator of Acorus gramineus in the Xianshan County, Guangdong Province, China (pers. comm., unpubl. data). The pollinator was determined by R. Gagné (pers. comm.). While depositing their eggs into the inflorescences the gall midges get sticky pollen onto their abdomen (see &), which they then transfer to the next plant’s stigmas when laying their next egg. The larvae of many species of this very large genus live and feed in flowers or catkins (R. Gagné, pers. comm.).

Dispersal

Not much is known of dispersal ecology but seeds and parts of rhizomes must be transported by water, because the plants are always growing in or near water. The European plants of A. calamus are triploid and therefore sterile, but have been naturalized there widely by rhizome pieces since the 16th century; the triploid plant found today in the eastern United States of America were introduced by early settlers as a medicinal plant and later became naturalized widely. The native diploids are found further inland and in Canada.

Morphology

Intravaginal squamulae are present in the leaf axils (as in almost all Helobiae and a few genera of Araceae). The leaves are distichous and unifacial (leaf blades), both surfaces being abaxial; only the base shows a bifacial development which can be interpreted as a sheath, and the upper part of the leaf clearly has the lower side completely connate to form a unifacial blade. Aerenchyma is present. The peduncle is triangular. Axillary branches develop from proleptic buds and begin with a cataphyllary 2-keeled prophyll, followed by foliage leaves. The continuation shoot arises in the axil of the leaf preceding the inflorescence. The last leaf of the flowering shoot (article) is considered as a spathe (following Engler), although it resembles a foliage leaf morphologically. The lower part of the spathe appears to be congenitally adnate to the axis of the inflorescence proper. Two separate vascular systems occur in the peduncle, one corresponding to the spathe and the other to the inflorescence. Ray (1988) rejected the interpretation that the leaf associated with the spadix is a spathe, preferring to consider it as a sympodial leaf. According to him, the shoot of A. calamus shows anisophyllous sympodial growth; the continuation shoot arises sylleptically and has a prophyll that resembles a foliage leaf.

Anatomy

Raphides and true laticifers are absent in the genus Acorus. Only irregular and tetrahedral crystals have been observed. Primitive vessels are present in the roots and rhizomes. The roots have large schizogenous intercellular spaces in the cortex. Idioblastic oil cells are present in the root cortex. The inner boundary of the stem (shoot, rhizome) is delimited by an endodermis with Casparian strips. The endodermis encircles the central cylinder, interrupted only where the leaf traces depart. The rhizome also has aerenchyma. The vascular bundles in the rhizome and leaves are amphivasal.

The leaves have a central region of large air cavities (aerenchyma) beneath a chlorenchyma of 3-5 layers of isodiametric cells. A chlorenchyma-less hypodermis is present. The leaf venation is parallel, with minor transverse veins joining the main, longitudinally oriented, system. This corresponds to the typical monocot venation type. Leaf variegation due to periclinal chimeras occurs in Acorus. Stomata are present on both surfaces and are brachyparacytic with one pair of subsidiary cells, the guard cells oriented parallel to the longitudinal axis of the leaf. Secretory cells (oil cells) are common and tannin cells are scattered in the leaves and rhizomes, occasionally also in the roots. Crystals are present as druses and prisms. The cuticle of the leaves is smooth or occasionally striate.

Taxonomy

Only one genus: The genus Acorus L. was early treated as a separate family, the Acoraceae, described in the year 1820. Later authors, like Schott 1858, 1860 and Engler 1889, 1905, treated this genus within the Araceae and for long afterwards it was considered as a member of this family until DNA analyses showed that Acorus is not related to the Araceae at all. It is the only genus of the family Acoraceae and the only family of the order Acorales. According to the DNA data, Acorus has a basal position within the monocots and is considered as the most ancient member of the monocots living today (Duvall et al. 1993).

Grayum (1987) pointed out morphological and anatomical characters which distinguish Acorus from the Araceae. These are: All Araceae (including the Lemnaceae = subfamily Lemnoideae) have raphides in their tissues, but Acorus lacks them. In Acorus, the flowering shoot consists of a peduncle adnate to a leaf as it is shown in two separate vascular bundles and the upper part of the leaf has been interpreted as the spathe which is different from the Araceae where the last leaf is modified into a spathe and the spadix is situated above it. The ensiform leaves of Acorus are unifacial, which is unknown in Araceae. In the Australian endemic, monotypic genus Gymnostachys R.Br. the leaves resemble those of Acorus in being ensiform, but they are bifacial; all other Araceae have leaves divided into a petiole and a well-developed blade. Acorus has seeds with perisperm and endosperm, but Araceae never have a perisperm. Other characters distinguishing Acorus from the Araceae are: bud trace patterns, presence of ethereal oil cells, stellate pattern of endothecial thickenings, introrse anthers dehiscence (but also introrse anthers in Zamioculcas), apical-axile placentation, exclusively axile vascular supply to the placentae, location and organisation of ovule hairs, 2-4 nuclei per tapetal cell, secretory anther tapetum, cellular endosperm development (Grayum 1987); Grayum also reported that the rust-fungus Uromyces sparganii Cooke & Peck, occurs in both Acorus calamus (in North America) and Sparganium.

Cytology

Acorus gramineus is known only in diploid populations (2n = 22, 24) and A. calamus in diploids (2n = 22, 24), reported from North America and rarely from Asia (Mongolia, middle Siberia), triploids (2n = 36) in Europe, Near East, India (Himalaya region) and Sakhalin, and in the eastern United States of America (introduced by early settlers there) and tetraploids (2n = 44, 48) which occur only in Asia. A plant from Borneo (Sarawak) has been shown to have 2n = 44 chromosomes (c. 44, certainly tetraploid, unpublished count); pollen of some specimens from New Guinea was studied and turned out to be sterile, suggesting that these plants were triploid and introduced, but it is possible that others are tetraploid (it is practically impossible to distinguish both cytotypes morphologically). The examined Bornean plant of A. calamus had somewhat narrower leaves (1 cm wide) and was formerly distinguished as var. angustifolius (Engler 1905), but a range of intermediates exist leading to the typical form. Triploid A. calamus is sterile and therefore never sets fruit. The triploid cytotype probably originated in the Himalayan region of India; it is widespread there and may have originated there from a cross between diploid and tetraploid cytotypes. The triploid A. calamus was brought to Turkey by people and in the 16th century to Central Europe as a medicinal plant, later to eastern North America; it was probably also taken to the Island of Sakhalin along with human migration.

Uses

Both species, A. calamus and A. gramineus, are used as medicinal and ornamental plants. Dried rhizomes of A. calamus (rhizoma calami) have been used for a very long time and were already known to ancient Egyptians, Greeks and Romans. Calamus oil (tinctura calami, extractum calami) was used for medicaments and is still today of some importance. The chopped rhizomes are still used as a tea to treat stomach ailments. Further use mainly of the rhizomes are for treatments of neurasthenia, chronic bronchitis, diarrhoea, abdominal distension, chills, colds, externally for abscesses, liver disturbance, antiulcer remedy, kidney and stomach as well as gut diseases; these uses are mainly reported from Chinese medicine. It is also said that β-asarone is cancerogenic (Keller & Stahl 1983). The rhizomes were also formerly used for perfume and are still used today to make a liqueur. Mostly the rhizomes of A. calamus are used, because the content of the ethereal oil is highest there, but the leaves are also used. Other reported uses are as an aromatizer for wine and tobacco and also as an insecticide. The bitter rhizomes of A. gramineus have also been used medicinally. Both species are commonly cultivated as ornamentals for wet sites, such as bog gardens, ponds and aquatic gardens and A. gramineus ‘Pusillus’, with leaves only up to 10 cm long, is used as an aquarium or terrarium plant. Cultivars of both species with variegated leaves are popular in gardens. Both species are hardy in temperate regions.

Embryology

Endosperm formation is cellular and unlike those types found in the Araceae. There is no chalazal basal apparatus. The anther tapetal cells are 2-4-nucleate.