Evolution Questions about Dicots vs Monocots

I know there are many features that distinguish dicots and monocots (flower petal multiples, leaf venation, root systems, etc). I was thinking about their evolution and have some questions I couldn’t find answers to online:

  1. First, among angiosperms, are there more monocots or dicots?

  2. I understand dicots evolved first. What are the benefits of monocot characteristics that enabled them to succeed alongside dicots?

  3. Do some environments favor one over the other?

  4. How have dicots managed to survive if monocots’ traits were competitive?

Thanks for any knowledge you can share on this puzzle!

I’m not scientist. In my opinion, Monocot = grass, some vines, palms. Dicots are big trees as well as herbaceous plants. It is hard to estimate the number of species or the quantity of the biomass in the two groups.
I’m thinking C3 and C4 carbon method of carbon accumulation, but I do not know exactly how these processes work. Anyway, grasses are fast growing in direct sun. Broad leaf saplings of trees grow slower in the initial stages. Some saplings are relatively fast growing too. If a land is cleared in my area, the first colonizers are grass, some sun-loving weeds, creeping vines. Then some shrubs may take over, follow by trees. Broad leaf trees will shade out the grasses. In the forest, palms grow tall or are climbing vines on big trees. Monocots do not grow very big. I can’t recall any species that is massive.

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I cannot answer all your questions, although I would like to comment on Q2.
Evolutionary botanists tend to agree that dicots arose first, and monocots are a specialised adaptation.
My old botany lecturer, the late Dr Cheung Low, together with a botanist from Germany had an alternative theory.
While conventional evolutionary botany teaches that in spite of their similar appearance palms and cycads are not related. Cycads are gymnosperms, related to the conifers. Palms, according to convention, are specialised monocots, possibly derived from grasses.
Dr Low however, contended that palms are derived from cycads and are possibly the ancestors of all angiosperms. This theory is based on the fact that pollen from the nypa palm has been found in Cretaceous sediments, a time when there is sparse evidence of other monocots. Nypa is a primitive type of tropical palm which grows in mangrove environments.


There are far more dicots than monocots.

Those who are purists about phylogeny (relationships) restrict the name Dicots to plants that evolved after the monocot/dicot split. (Just like monocots are plants that evolved after that split.) Some of the more primitive plants, like Magnolias, are, technically neither monocots nor dicots. I prefer to ignore that and use the term dicots for all non-monocot flowering plants. (Just letting you know I’m wrong to do that.)

It is thought that the earliest monocots were aquatic plants. They had little need for well organized water transport, no real need for deep taproots, apparently no need for well-developed leaf blades, certainly no need for much support since the water would hold them up. So they lost or reduced all those expensive tissues and organs. Leaves were reduced to petioles, which would become sheaths. When conditions changed, similar selection pressures led monocots to solve the same basic issues that dicots had already solved. Populations that lived in drier conditions re-evolved leaf blades and sturdier, more effective systems of transport and support. They did it differently than the dicots had always done it; parallel leaf veins, scattered vascular bundles, etc.

To this day, monocots tend to occupy wetter habitats than dicots, but that’s only a trend. There are lots of exceptions.

The competition isn’t between monocots as a whole or dicots as a whole, but between individuals and populations that happen to be monocots or dicots. What traits are most successful depend on details of habitats and ecosystems. I’m not sure you can generalize about their success at the level of these two major groups. You also have no reason to expect that all monocots will outcompete all dicots, or visa versa.

Well, in one way you can. Because of their scattered vascular bundles, monocots are very, very bad at secondary growth (growing wider). They don’t make good trees. Oh, there are palms, but palms generally don’t produce side branches and they come out of the ground as wide as they’ll ever be. They have to have very large leaves to reach sideways from the trunks and there are limits to what those leaves can accomplish in terms of claiming 3-D space. In general, monocots don’t make good shrubs, either, though Pandanus and a few others do an adequate job of it. Dicots really don’t have to worry about monocots evicting them from the woody plant niches! Restrained by their vascular system, monocots don’t even do really well in the niches for branching herbaceous plants – in general.

Monocots shine at growth forms that don’t require secondary growth. They can grow old by becoming longer rather than by growing wider. They do well as geophytes that make bulbs and send up yearly flowering shoots then move their perennial growing point to the side or up to make a new bulb. They shine is at being rhizomatous (or secondarily forming bunchgrasses by minimizing their rhizomes). These rhizomatous plants spread out under the ground and send up shoots that don’t branch much and don’t last long. Only the rhizome is long-lived. (In bunchgrasses, the “crown,” the mass of very very short rhizomes is perennial.) Think grasses, sedges, and rushes as well as rhizomatous “lilies” like Solomon’s Seal and trilliums. Very, very successful plants that don’t have the same life style as branching, leafy, often woody herbaceous plants. Grasslands and shrub-steppe are stable plant communities where diverse dicots can live with diverse monocots.


By the way, C4 and CAM photosynthesis have evolved repeatedly in both monocots and dicots. (C3 is ancestral.)


My first instinct is to say that Agaves & friends are an exception, but I could be interpreting their biology wrong? I see them as plants that grow wider than longer, but maybe you covered that in the bunchgrass section.

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Yeah I suppose you could say this about many Asparagales, since Asparagus also do quite some branching. I think the point here is that even agaves still rely on a central stem and a small root system that support vertical growth better than horizontal growth, even if their leaves and stems grow larger, rather than on elaborate branches and ever thickening woody stems.


No, I think you’re right. I have the vague shadow of a memory that their secondary growth is kind of odd, compared to dicot secondary growth, but they have it.


Possibly related: the primary staple food crops in the temperate zone are monocots (wheat, rice, maize, all grasses); whereas in the humid tropics, some are monocots (bananas/plantains, cocoyams) but at least as many are dicots (cassava, sweet potato).

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Strelitzia nicolai
and bamboo

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