Classification of Aristolochiales: 3 Families | Dicotyledonae

In this article we will discuss about the classification of Aristolochiales. According to Engler, Aristolochiales consists of three families:- 1. Aristolochiaceae 2. Rafflesiaceae 3. Hydnoraceae.

Family # 1. Aristolochiaceae:

Aristolochiaceae are herbs or shrubs, climbing or creeping, rarely attaining tree-size; usually more or less foetid. Leaves simple, alternate, exstipulate, with ethereal oil-glands.

Flowers of Aristolochiaceae is solitary or in cymose clusters or racemes or spikes, terminal or axillary; bisexual, zygomorphic or actinomorphic, epigynous, bracteate or not. Perianth usually in one, whorl, petaloid, tubular, 3-lobed or with an oblique mouth; tube contracted in the middle, inflated below and dilated at the mouth; lobes valvate.

Stamens of Aristolochiaceae is 6, often much more, very rarely 5, in one or more whorls, free or adnate to the style; filaments short and thick or the anthers sessile; anthers erect with parallel cells dehiscing longi­tudinally and dorsally; pollen uniaperturate.

Carpels of Aristolochiaceae is 6 or 4, united to form a syncarpous ovary; ovary inferior, with cells as many as carpels; style columnar; stigma lobed; ovules anatropous, usually many on axile or parietal placentas. Fruit a septicidal cap­sule or a berry; seeds with copious fleshy endosperm and minute embryo.

Flowers of Aristolochiaceae are insect pollinated. In Aristolochia the jug-shaped perianth keeps the insect captive for some-time. The narrow middle portion of the perianth tube is beset with stiff hairs which point downwards. These prevent the insect to climb up and run away. After sometime the hairs wither and drop down and the insect is freed from captivity.

While inside the perianth-tube the insect moves about in the basal inflated portion of the perianth and gets itself dusted with pollens which it carries on its body and when visiting another flower pollinates the stigma. The mouth of the perianth tube is of various shapes and with peculiar colouring and attracts the insects which it traps for pollination.

In Asarum the flower is actinomorphic and there is an inner whorl of scale-like perianth which represents the corolla. In Saruma the petals and the carpels are free and the fruit is a cluster of follicles: Species of the genus Apama are small trees.

The Aristolochiaceae family contains 8 genera and about 600 species and are found in temperate and tropical region of northern hemisphere. In India it is represented by several species of Aristolochia, e.g. A. indica Linn., A. acuminata Lam., A. saccata Wall, A. Roxburghiana Klotz, etc. and Apama Wallichii R. Br.

The family is of no economic importance but a few are cultivated in gardens as ornamental plants and also as curiosities due to the shape of the flowers, e.g. Aristolochia grandiflora Sw.—the Pelican flower, A. drurior Hill—the Dutchman’s pipe.

The Aristolochiaceae family was included in Aristolophiales along with Rafflesiaceae and Hydnoraceae by Engler, Rendle and others but modern taxonomists think that these 3 families are not closely related and not to be included in the same order.

It is consi­dered that Aristolochiaceae has been derived from Magnoliales or Ranales and its relationship might be with Berberidaceae or Lardizabalaceae. Anatomical characters also show some similarities between Aristolochia and Berberis.

Family # 2. Rafflesiaceae (Cytinaceae):

Rafflesiaceae are leafless root—rarely stem-parasites with a mycelium-like body; the vegetative body consisting of network of filiform cellular structure remains within the host plant.

The flowers of Rafflesiaceae come out through the intercellular space of the root of the host and are solitary; unisexual, monoecious or dioecious, epigynous, actinomorphic and with one whorl of perianth. Perianth sepaloid or petaloid, with 4-6 segments which are free or basally connate.

A disc is often present in the centre. Stamens many in the male flower and the sessile anthers are inserted on the lower side of the disc; anthers bursting by apical pores or by longitudinal slits, pollen often viscuous.

Ovary inferior or semi- inferior, 1-locular or apparently many-locular due to the placentas reaching almost the centre of the cell; stigma discoid or lobate or stigmas many on top of ovary; ovules many on parietal placentas or pendulous from the apex of ovary-chamber; integument one only. Fruit a berry, often dehiscing irregularly; seed with an undifferentiated embryo surrounded by one layer of oily endosperm cells.

The Rafflesiaceae family consists of 8 genera and about 50 species widely distributed in the tropical region. The flowers in this family are usually large and particularly in Rafflesia they attain a great size. In R. Arnoldi R. Br. the flower is not less than a metre across. The Rafflesiaceae family is represented in India by a single species, viz. Sapria himalayana Griff.

Family # 3. Hydnoraceae:

Hydnoraceae are leafless root parasites with a rhizome. The rhizome bears large fleshy solitary flowers that come above ground. Flowers of Hydnoraceae is bisexual, epigynous, with one whorl of perianth. Perianth segments 3-4, thick and fleshy, united below in a tube. Stamens in Hydnora as many as perianth-lobes with a large number of pollen sacs.

In Prosopanche stamens 3, united to form a covering over the stigma and 3 staminodes. Ovary inferior, uni­locular; ovules many, orthotropous, on parietal placentation; integument 1 or absent. Fruit baccate with minute seeds; endosperm copious.

Hydnoraceae is a small family with 20 species under 2 genera, viz., Hydnora in Africa and Prosopanche in Argentina.

Hydnoraceae is closely related to Rafflesiaceae. It is not economically important.