Monocotyledon

Monocotyledons (/ˌmɒnəˌkɒtəˈliːdənz/), commonly referred to as monocots, (Lilianae sensu Chase & Reveal) are flowering plants whose seeds contain only one embryonic leaf, or cotyledon. A monocot taxon has been in use for several decades, but with various ranks and under several different names. The APG IV system recognises its monophyly but does not assign it to a taxonomic rank, and instead uses the term "monocots" to refer to the group.

Monocotyledons are contrasted with the dicotyledons, which have two cotyledons. Unlike the monocots however, the dicots are not monophyletic and the two cotyledons are instead the ancestral characteristic of all flowering plants. Botanists now classify dicots into the eudicots ("true dicots") and several basal lineages from which the monocots emerged.

The monocots are extremely important economically, culturally, and ecologically, and make up a majority of plant biomass used in agriculture. Common crops such as dates, onions, garlic, rice, wheat, maize, and sugarcane are all monocots. The grasses alone cover over 40% of Earth's land area and contribute a significant portion of the human diet. Other monocots, like orchids, tulips, daffodils, and lilies are common houseplants and have been the subjects of several celebrations, holidays, and artworks for thousands of years.

The monocots have, as the name implies, a single (mono-) cotyledon, or embryonic leaf, in their seeds. Historically, this feature was used to contrast the monocots with the dicotyledons or dicots which typically have two cotyledons; however, modern research has shown that the dicots are paraphyletic. From a diagnostic point of view the number of cotyledons is neither a particularly useful characteristic (as they are only present for a very short period in a plant's life), nor is it completely reliable. The single cotyledon is only one of a number of modifications of the body plan of the ancestral monocotyledons, whose adaptive advantages are poorly understood, but may have been related to adaption to aquatic habitats, prior to radiation to terrestrial habitats. Nevertheless, monocots are sufficiently distinctive that there has rarely been disagreement as to membership of this group, despite considerable diversity in terms of external morphology.

With over 70,000 species, monocots are extremely evolutionarily successful and occupy a diverse set of niches: Perennial geophytes including orchids (Asparagales); tulips and lilies (Liliales); rosette and succulent epiphytes (Asparagales); mycoheterotrophs (Liliales, Dioscoreales, Pandanales), all in the lilioid monocots; major cereal grains (maize, rice, barley, rye, oats, millet, sorghum and wheat) in the grass family; and forage grasses (Poales) as well as woody tree-like palm trees (Arecales), bamboo, reeds and bromeliads (Poales), bananas and ginger (Zingiberales) in the commelinid monocots, as well as floating or submerged aquatic plants such as seagrass (Alismatales) are all monocots.

The most important distinction is their growth pattern, lacking a lateral meristem (cambium) that allows for continual growth in diameter with height (secondary growth), and therefore this characteristic is a basic limitation in shoot construction. Although largely herbaceous, monocots include some arboraceous species that reach great height, length and mass. The latter include agaves, palms, pandans, and bamboos. This creates challenges in water transport that monocots deal with in various ways. Some, such as species of Yucca, develop anomalous secondary growth, while palm trees utilise an anomalous primary growth form described as establishment growth (see Vascular system). The axis undergoes primary thickening, which progresses from internode to internode, resulting in a typical inverted conical shape of the basal primary axis (see Tillich, Figure 1). The limited conductivity also contributes to limited branching of the stems. Despite these limitations a wide variety of adaptive growth forms has resulted (Tillich, Figure 2) from epiphytic orchids (Asparagales) and bromeliads (Poales) to submarine Alismatales (including the reduced Lemnoideae) and mycotrophic Burmanniaceae (Dioscreales) and Triuridaceae (Pandanales). Other forms of adaptation include the climbing vines of Araceae (Alismatales) which use negative phototropism (skototropism) to locate host trees (i.e. the darkest area), while some palms such as Calamus manan (Arecales) produce the longest shoots in the plant kingdom, up to 185 m long. Other monocots, particularly Poales, have adopted a therophyte life form.

The cotyledon, the primordial Angiosperm leaf consists of a proximal leaf base or hypophyll and a distal hyperphyll. In monocots the hypophyll tends to be the dominant part in contrast to other angiosperms. From these, considerable diversity arises. Mature monocot leaves are generally narrow and linear, forming a sheathing around the stem at its base, although there are many exceptions. Leaf venation is of the striate type, mainly arcuate-striate or longitudinally striate (parallel), less often palmate-striate or pinnate-striate with the leaf veins emerging at the leaf base and then running together at the apices. There is usually only one leaf per node because the leaf base encompasses more than half the circumference. The evolution of this monocot characteristic has been attributed to developmental differences in early zonal differentiation rather than meristem activity (leaf base theory).

The lack of cambium in the primary root limits its ability to grow sufficiently to maintain the plant. This necessitates early development of roots derived from the shoot (adventitious roots). In addition to roots, monocots develop runners and rhizomes, which are creeping shoots. Runners serve vegetative propagation, have elongated internodes, run on or just below the surface of the soil and in most case bear scale leaves. Rhizomes frequently have an additional storage function and rhizome producing plants are considered geophytes (Tillich, Figure 11). Other geophytes develop bulbs, a short axial body bearing leaves whose bases store food. Additional outer non-storage leaves may form a protective function (Tillich, Figure 12). Other storage organs may be tubers or corms, swollen axes. Tubers may form at the end of underground runners and persist. Corms are short lived vertical shoots with terminal inflorescences and shrivel once flowering has occurred. However, intermediate forms may occur such as in Crocosmia (Asparagales). Some monocots may also produce shoots that grow directly down into the soil, these are geophilous shoots (Tillich, Figure 11) that help overcome the limited trunk stability of large woody monocots.