SUMMARY
More than 400 species of vascular plants, in 87 genera, are acholophyllous and heterotrophic, but not directly parasitic upon autotrophs. They are usually, but incorrectly, described as 'saprophytes’since they are in fact nourished by means of specialized mycorrhizal associations. Although distributed world-wide, they are most abundant and show the greatest species-richness in the Neotropics and Palaeotropical regions. Their aerial parts range in size from a few centimetres to extensive liane types up to 40 m long. With few exceptions, their habitats are dense moist forests in which there is a surface accumulation of leaf litter, often in situations which are too shaded for autotrophic growth.

Although the achlorophyllous mycorrhizal mode of life has evolved independently many times and in widely disparate taxonomic groups, such plants show strong convergent evolution in particular adaptations to their peculiar mode of life. Most prominant amongst these are reductions in the size of seed and embryo, and the lack of differentiation of the embryo at maturity. The number of seeds produced by each flower is typically very large and the shape, structure and surface features of seeds involving adaptation for wind dispersal show remarkable parallels in many species. Specific adaptations for zoochory are rare but well developed in a small number of genera, some of which produce scents like fungal fruit bodies or floral parts which mimic fungal sporocarps.

Vegetative parts are often even more conspicuously reduced. Most myco-heterotrophs are entirely subterranean for most of their lives and these stages exhibit adaptations consistent with a change in function from organs of absorption to organs of storage, shown by the almost universal loss of root hairs, decrease in surface area as exhibited in short cylindric‘vermiform’and tuberous roots or, in extreme cases, the complete suppression of roots and the formation of a swollen tuber or rhizome. Increased width of the root cortex often accommodates mycorrhizal infection and stores of carbohydrates and other materials obtained from the fungal symbiont. Mycorrhizal infection is confined to the below-ground parts of the plants but may be found there in modified stems as well as in roots.

In many genera, stems are exceptionally slender and thread-like and their vascular tissues are either reduced to a single narrow cylinder of bicollateral bundles or, minimally, to four or six narrow bundles in the cortex. Secondary thickening is poorly developed in all but a tiny minority of species, lignification being confined to annular or, rarely, a few scalariform xylem vessels. Phloem is present in very small amounts and then mainly as parenchyma with sieve tubes frequently recorded as narrow and possibly with abherent sieve plates.

Leaves are typically reduced to widely spaced achlorophyllous scales on the inflorescence axis. Occasionally, they are present only on underground rhizomes or tubers. The vascular supply to the leaf-scales, normally reduced to a single trace, may be absent. Vestigial stomata are sometimes found on leaves and, in a few species which retain traces of chlorophyll, on shoots but, in most fully heterotrophic species, stomata are absent from aerial parts.

Since their seeds are very small and contain minimal reserve carbohydrates, the germination of myco-heterotrophs in nature would appear to depend upon infection by an appropriate symbiotic fungus at an early stage. The nature of the carbohydrates transferred from the fungus to the plants has not been determined. Once acquired from The fungal partner, most plants store carbon in a variety of forms, the most common of which is starch, although other compounds including glucomannan, fructan and calcium oxalate art important in some specks.

Asexual reproduction is frequently important with root tubers, tubercles and rhizomes providing the means of vegetative spread. Nonetheless, all the angiospermous species recorded to date also reproduce sexually. Floral structures show varying degrees of reduction concomitant with myco-heterotrophy. Inflorescences are typically small, often with a single terminal flower, and the floral parts often show extreme simplification, with the production of unilocular or, more rarely, bilocular and trilocular ovaries. In some of the most highly adapted species, there is reduction of integumentary layers surrounding each ovule from the normal bitegmic condition to unitegmic or, occasionally, ategmic. With the principal exception of the Monotropaceae, placentation is typically trimerous and parietal. Flowers normally appear to be cross-pollinated and are brightly coloured. nectiferous, occasionally scented, and can demonstrate extreme morphological adaptations which attract insects as in the production of lone caudate tepals or fungus-mimicking structures.

Much is still to be learned about the adaptive features and especially about the physiology of these plants and of their early developmental stages during which the essential associations with fungi are established. Similarly, studies of the taxonomy and physiology of most of their fungal partners are still in their infancy.

The biology of myco‐heterotrophic (‘saprophytic’) plants

The function of MADS-box genes in flower and fruit development has been uncovered at a rapid pace over the past decade. Evolutionary biologists can now analyse the expression pattern of MADS-box genes during the development of different plant species, and study the homology of body parts and the evolution of body plans. These studies have shown that floral development is conserved among divergent species, and indicate that the basic mechanism of floral patterning might have evolved in an ancient flowering plant.

Function and evolution of the plant MADS-box gene family

A phylogenetic analysis of the monocots was conducted on the basis of nucleotide sequence variation in two genes (atpA, encoded in the mitochondrial genome, and rbcL, encoded in the plastid genome). The taxon sample of 218 angiosperm terminals included 177 monocots and 41 dicots. Among the major results of the analysis are the resolution of a clade comprising four magnoliid lineages (Canellales, Piperales, Magnoliales, and Laurales) as sister of the monocots, with the deepest branch within the monocots between a clade consisting of Araceae, Tofieldiaceae, Acorus, and Alismatales, and a clade that includes all other monocots. Nartheciaceae are placed as the sister of Pandanales, and Corsiaceae as the sister of Liliales. The Triuridaceae, represented by three genera, including Lacandonia, are resolved as monophyletic and placed in a range of positions, generally within Pandanales. Dasypogonaceae and Arecaceae diverge sequentially from a clade that includes all other commelinid taxa, and within the latter group Poales s. lat. are sister of a clade in which Zingiberales and Commelinales are sisters. Within Poales s. lat., Trithuria (Hydatellaceae) and Mayaca appear to be closely related to some or all elements of Xyridaceae. A comparison was conducted of jackknife and bootstrap values, as computed using strict-consensus (SC) and frequency-within-replicates (FWR) approaches. Jackknife values tend to be higher than bootstrap values, and for each of these methods support values obtained with the FWR approach tend to exceed those obtained with the SC approach.

A Phylogeny of the Monocots, as Inferred from rbcL and atpA Sequence Variation, and a Comparison of Methods for Calculating Jackknife and Bootstrap Values

Flowers evolved in many steps, probably starting long before flowering plants (angiophytes) originated. Certain parts of flowers are conservative and have not changed much during evolution; others are evolutionarily highly plastic. Here conservative features are discussed and an attempt is made to trace them back through their evolutionary history. Microsporangia and ovules (which develop into seeds) are preangiophyte floral elements. Angiospermy, combined with postgenital fusion, was the most prominent key innovation in angiophytes. Angiospermy and thecal organization of stamens originated earlier than all clades of extant angiosperms (the crown group of angiophytes). Differentiation of a perianth into calyx and corolla and syncarpy appeared after the first branching of the basalmost clades of extant angiosperms. Sympetaly and floral tubes as well as tenuinucellar, unitegmic ovules originated as major innovations in the clade that led to asterids. An obvious trend in flower evolution is increased synorganisation of parts, which led to new structures. Fixation of floral organ number and position was a precondition for synorganization. Concomitantly, plasticity changed from number and position of organs to shape of the new structures. Character distribution mapped onto cladograms indicates that key innovations do not appear suddenly, but start with trials and only later become deeply rooted genetically in the organization. This is implied from the common occurrence of reversals in the early history of an innovation. J. Exp. Zool. (Mol. Dev. Evol.) 291:105-115, 2001.

Origins of flower morphology

Gynoecium diversity and systematics in basal monocots

Anatomia reproductiva de Lacandonia schismatica (Lacandoniaceae).

Reproductive anatomy of lacandonia-schismatica (lacandoniaceae)

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Anatomia reproductiva de Lacandonia schismatica (Lacandoniaceae).

Reproductive anatomy of lacandonia-schismatica (lacandoniaceae)