The Monocot Class of Flowering Plants

by Bob Harms ()
The online Oxford English Dictionary presents the following:
monocot
shortened forms of monocotyledon
monocotyledon
A flowering plant having one cotyledon or seed-leaf in the embryo.
cotyledon
The primary leaf in the embryo of the higher plants ...; the seed-leaf. The number of cotyledons in the seed serves as an important basis of classification of angiosperms into monocotyledons... with one cotyledon, and dicotyledons... with two; in gymnosperms the number varies, being usually more than two.
Thus, the embryos of seed-bearing plants have cotyledons. There are two essential differences between the gymnosperms, such as pines and junipers, and the flowering plants (angiosperms). Determining all this is easier said than done, as should be clear from a comparison of similar-looking 'fruits' from (1) the gymnosperm Juniperus ashei (Ash juniper), (2) the monocot Smilax bona-nox (greenbriar) and (3) the dicot Prunus serotina (escarpment cherry):
  

The essential difference here is that the 'fruity' portion of the juniper berry developed from the fusion of the scales surrounding the bare seed; that of the others, from a flower's ovary. But the seeds of all three contain one or more cotyledons.

'Distinguishing' Features of the Monocots

The OED definition, if not outdated, is greatly oversimplified. No single feature suffices to distinguish monocots from dicots. Monocots have evolved from a branch within the dicot class, largely by simplification of one or more dicot structures. And given dicots may have independently undergone convergent simplifications similar to those of the monocots; e.g., a single cotyledon or parallel veins.

Below I list traits associated with monocots, but each of these all must be qualified by "in general." They may not necessarily hold for any individual family or genus.

  1. Monocots have one cotyledon.
  2. Monocot leaves tend to have parallel veins; in dicots the veins are netted.
  3. Monocot floral parts are in multiples of 3; dicots are based on 4's or 5's.
  4. The vascular bundles in monocots stems are scattered; in dicots they form a ring surrounding the pith.
  5. Monocot root systems are adventitious; with dicots both primary and adventitious root systems occur.
  6. Monocot pollen has a single furrow or pore through the outer layer (monosulcate); dicot pollen is mostly with three furrows or pores.
  7. Monocots lack secondary growth leading to woody plants; dicots are not so restricted.

1. Cotyledons as Distinguishing Features of the Monocots

Shown below are seedlings with cotyledons from two gymnosperms, Ashe juniper - with 2 - and Taxodium distichum (bald cypress) - with a band of numerous cotyledons.
  

Four dicotyledons shown are Parkinsonia aculeata (retama), Sophora secundiflora (Texas mountain laurel), Chilopsis linearis (desert willow) and Arbutus xalapensis (Texas madrone). The cotyledons of S. secundiflora remain below the surface (are 'hypogeous'). Those of C. linearis are deeply cleft, almost appearing as four. Note that in all of these images the true vegetative leaves are above the cotyledons and have a different shape or texture. But if the cotyledons rise above the surface, chlorophyll is produced, with photosynthesis much as with true leaves.

   

Although nearly everyone who has raised plants from seed is familiar with the pairs of sprout leaves that accompany the germination of dicots. The cotyledon of a monocot is not so easily observed, and not just because it remains below the surface (as with dicot Texas mountain laurel). The literature on monocot cotyledons is generally uninformative if not misleading. The University of Kentucky's College of Agriculture site Plant Physiology I, Lecture 8: Seed and fruit development provides an explanation that seems to account for the difficulties I have encountered searching for the cotyledons of monocots:

The single cotyledon of the monocots (the scutellum Fig. 1, 3A) is usually of a secretory and absorptive nature, never exiting the seed proper, even after germination is complete. It abscises from the seedling and is shed along with the exhausted endosperm and testa upon the completion of seedling establishment. It rarely contains substantial amounts of stored reserves being associated with endospermic seeds (seeds in which the major storage organ is an endosperm).
In conjunction with grasses, we frequently encounter statements such as that in F. G. Gould, Grass Systematics (1968; p. 362):
Scutellum. Haustorial tissue of the embryo located between the endosperm and the plumule-radicle axis of the embryo; considered by some to be the grass cotyledon. [emphasis by Bob Harms]
Two series of grass germination images, for Sporobolus clandestinus (rough dropseed) and Setaria leucopila (yellow foxtail), are given below. The scutellum, visible with the second set, has been labeled. Shown in both is the 'rapid elongation of the embryonic shoot and emergence of the coleoptile' (Gould, p. 29), a sheath from which the true leaf can be seen emerging.

Although no monocot has two cotyledons, in the rudimentary embryo of orchids, consisting of only a few cells, the process of simplification seems to have reached a stage of no cotyledon at all.

In sum, the monocot cotyledon appears to be a phylogenetic issue likely to elude the amateur botanist hoping to observe it.

2. Parallel veins of monocot leaves.

Monocot leaves generally have parallel veins, as shown for Sorghum halepense (Johnsongrass), Cladium mariscus (Jamaica sawgrass) and Sabal minor (dwarf palmetto):
  

Some monocots are said to have netted veins, such as Smilax (Correll and Johnston 1970, p. 409), but in both Smilax tamnoides (bristly greenbrier) and Potamogeton illinoensis (Illinois pondweed), shown below, one senses that the reticulation is superimposed on a primary parallel vein pattern.

 

The netted veins of dicots are quite different as my be noted with Ipomoea lindheimeri (blue morning glory), Funastrum cynanchoides (fringed twinevine) and Liatris mucronata (narrowleaf gayfeather):
  

3. Monocot floral parts in multiples of 3

With grasses this feature seems to be restricted to the number of stamens, as shown with Bromus catharticus (rescuegrass) floret. In contrast the bromeliad Tillandsia recurvata (ballmoss) has its flower parts in 3s; e.g. the 3 petals shown.
 

The lily Schoenocaulon texanum (green lily) has 2 whorls of 3 stamens (left), 3 styles (right), and 2 whorls of essentially identical sepals and petals.

Iris hexagona (Dixie iris) reveals its flower parts to be in multiple whorls of 3.

The monocot Potamogeton illinoensis (Illinois pondweed) deviates from the expected pattern with 4 sepal-like segments, 4 stamens.

Typical among the dicots are Rorippa nasturtium-aquaticum (watercress) with 4s, and Gilia rigidula (bluebowls), here dissected, with 5s.

To illustrate an exceptional dicot, a tripartite floral structure is the rule with Aristolochia serpentaria (Virginia dutchmanspipe), dissected to expose its 6 stamens and 3-lobed style

4. Vascular bundles in monocot stems.

Contrast the scattered vascular bundles of Tripsacum dactyloides (eastern gamagrass) with the outer ring of six in the stem of dicot Justicia americana (American water-willow):
 

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