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

Aquatic species represent fewer than two percent of all flowering plants, and only 18 aquatic genera have acquired true hydrophily (water-pollination) which is associated with an unusually high incidence of unisexual flowers. From the subset of submersed, hydrophilous angiosperms, only 13 genera have colonized marine habitats. The evolution of hydrophily, unisexuality, and marine habit in angiosperms was explored using estimates of phylogeny obtained by phylogenetic analyses of chloroplast (rbcL) gene sequence data. Despite what might appear to be difficult evolutionary transitions, hydrophiles are highly polyphyletic with independent origins in the monocotyledon subclass Alismatidae in addition to two derivations in the dicotyledon families Ceratophyllaceae and Callitrichaceae. Yet, even in alismatids, hydroph- ily has evolved many times. Unisexuality has also evolved repeatedly in the Alismatidae, and is ancestral to the evolution of hydrophiles and marine plants in the Hydrocharitaceae. Marine angiosperms (known only from Alismatidae) have evolved in three separate lineages. The multiple origins of hydrophilous, marine plants offer an extraordinary example of convergent evolution in angiosperms.

http://www.jstor.org/stable/pdfplus/2419820.pdf

Phylogenetic Studies in Alismatidae, II: Evolution of Marine Angiosperms (Seagrasses) and Hydrophily

We have amassed structural data for 108 characters and 52 taxa of magnoliids and basal monocots and eudicots, including observations by P. K. Endress and A. Igersheim on flowers. These data were analyzed separately and in combination with rbcL, 18S, and atpB sequences. Besides confirming agreements between previous analyses of both kinds of data (reduction of Magnoliales to six families; relation of Piperaceae, Saururaceae, Lactoris, and Aristolochiaceae), trees based on this data set show shifts toward molecular results (separation of Illiciaceae and Schisandraceae from Winteraceae and of Amborella, Austrobaileya, Trimeniaceae, and Chloranthaceae from Laurales; relation of Winteraceae and Canellaceae) plus continued conflicts (association of Chloranthaceae with Amborella and Trimeniaceae and of Nymphaeales with monocots). In cases where molecular and morphological trees conflict, combined analyses of morphological and molecular data generally yield the same topologies as molecular analyses, but morpholog...

https://www.jstor.org/stable/10.1086/317578

Morphological Phylogenetic Analysis of Basal Angiosperms: Comparison and Combination with Molecular Data

It appears that the tapetum is universally present in land plants, even though it is sometimes difficult to recognize, because it serves mostly as a tissue for meiocyte/spore nutrition. In addition to this main function, the tapetum has other functions, namely the production of the locular fluid, the production and release of callase, the conveying of P.A.S. positive material towards the loculus, the formation of exine precursors, viscin threads and orbicules (= Ubisch bodies), the production of sporophytic proteins and enzymes, and of pollenkitt/tryphine. Not all these functions are present in all land plants:Embryophyta. Two main tapetal types are usually distinguished in theSpermatophyta: the secretory or parietal type and the amoeboid or periplasmodial type; in lower groups, however, other types may be recognized, with greater or lesser differences. A hypothetical phylogenesis of the tapetum is proposed on the basis of its morphological appearance and of the nutritional relations with meiocytes/spores. The evolutionary trends of the tapeta tend towards a more and more intimate and increasingly greater contact with the spores/pollen grains. Three evolutionary trends can be recognized: 1) an intrusion of the tapetal cells between the spores, 2) a loss of tapetal cell walls, and 3) increasing nutrition through direct contact in narrow anthers.

The tapetum: its form, function and possible phylogeny in Embryophyta

Calcium Oxalate Crystals in Monocotyledons: A Review of their Structure and Systematics

The monocotyledons :a comparative study , The monocotyledons :a comparative study , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

H. T. Clifford

Papers

The monocotyledons: a comparative study.