Monocots versus Dicots- what's the big deal?
In some fairly non-scientific articles I will uncover and explain some of the taxonomic differences in plants that somehow totally went over my head in college (when I was SUPPOSED to be learning this stuff). Of course, school is wasted on the students, (like youth being wasted on the young). I went through a grueling gazillion years of schooling in the process of maturation and preparation for being kicked out into the world. Never in all those years did I think I would use any of that silly stuff they crammed down my throat in school. Only now I have a hobby that has become an obsession focusing on weird plants and NOW I want to really learn things that I "learned" in school eons ago. It is too bad I can't got back and do it all over again, knowing what I know now. And so I figure you might want to learn them, too.
Typical monocot examples (bromeliad- Neoregelia species, on left; and an agave (Agave bracteosa) left) showing long, linear leaves and radial symmetry
Typical dicots (Acacia tree left and George Burns Rose right) showing complex branching and secondary growth
To my mind, one of the most arbitrary distinctions in the plant world is the separation of flowering plants by whether they are monocots or dicots. What are these first of all? A monocot, as the name suggests, has one ‘cot' or cotyledon. A cotyledon is basically the first leaf that sprouts from a seed. Monocots have one, and dicots have two. Big deal. Some plants have one leaf to start their lives out with, and some have a pair. Seems like too little a difference to base an entire plant classification system upon. But then, I suppose we have to start somewhere. And since this is a difference early on in a plant's life, maybe it is a basic difference that means something? I mean, there are some plants that don't even have a ‘cot'. Ferns have no cot; neither do pine trees. It turns out lots of other plants have no cot... but all the plants that make flowers (aka angiosperms) have either one cot or two.
Typical palm seedlings coming up as single leaves (monocot)- left; right is a Wikipedia shot of a dypical dicot showing the first embyronic leaves followed by the more typical mature leaves (right)
Another Wikipedia photo comparing the two:monocot on left, dicot on right
Conifers don't fit into either category, being non-flowering plants. These plants make cones instead (Montery Cypress left and youing cones of a Vanderwulf's Pine on right)
Encephalartos ferox cone (left) is another example of a plant that is neither a monocot or dicot, but a coning plant instead, despite is 'palm-like' appearance; right is a Blechnum brasiliense, a fern also showing radial symmetry, but not a monocot or dicot, either. These plants reproduce by spores- no flowers, either.
The cotyledon is the plant embryo's version of a leaf, by the way... not really a true leaf. The embryo in the seed pops out a leaf and this leaf absorbs nutrients for the developing seedling until the seedling has grown big and strong enough to makes it first ‘real' leaves.
Well, as it turns out, this itty bitty little seemingly inconsequential, almost random difference leads to a whole lot of more impressive differences down the road. Not every characteristic of a monocot is necessarily unique to monocots, nor those of dicots. But overall, these two plant groups have a bunch of very different characteristics from the way they grow, the way they transport food and water, and most importantly (from my point of view), the way they look. Who would have thought that starting out with one leaf versus two would end up making these plants so different?
After looking over my collection of favorite plants, most turn out to be monocots. Monocots are really simpler plants relative to dicots. I guess I like simple plants. I think that makes them more primitive, too, which is no surprise. I like ferns and cycads, two even more primitive plants. But really it is the symmetry of the plants that attracts me so much. Monocots tend to continue growing the way they started, one leaf at a time, coming from the center of the plant or apical meristem. So if you see a plant (few exceptions of course, as there always are) in which long, strap-like leaves with parallel veins erupt from the center forming a rosette- that is probably a monocot. The plants that come to mind in my own garden are the aloes, agaves, yuccas, dracaenas, bromeliads, iris and lilies, palms, grasses and bamboo (that last one seemed a bit less obvious to me at first).
One can see just by the way these two variegated agaves are striped that they are monocots, aside from knowing anything related to an agave will be a moncot. The stripes follow the growing shape of the leaves, which all grow out from the center apical meristem. Dicot leaves don't usually grow so linearly
This Beaucarnea recurvata variegate also emphasizes the linear direction of the leaves growth (left); though not variegated, the Dasylirion wheeleri on the right also shows a typical radial symmetry and long, strap-like leaves all coming from a central apical meristem
Haworthias and Aloes and all their relative are also monocots, and grow typically for them (Haworthia retusa left, Aloe 'Red Ridge' right)
Before I planted up the back yard with all my potted monocots, I had to get rid of the most common monocot of all- the lawn (a typical monocot)
Perhaps more obviously a monocot due to its radial clumping nature, Fescue grass, like all monocots, starts out life as single blade of grass, which is actually the mono-cotyledon
Bamboo is less obviously a monocot until you look at its relatively non-tapering culms and the fact it's related to grass, which is a typical monocot (left); New Zealand Flax, a common landscape plant and a plant that even looks like a huge grass clump, is another fairly obvious monocot (right)
Not only do monocot leaves tend to rise from a central point in the plant (or points for those monocots that do indeed 'branch dichotomously), the leaves tend to be connected to the central structure of the plant differently than do dicot leaves. Monocot leaves often attach by a sheath which surrounds the central stem or trunk, reflecting, again, how it was created to begin with. Dicot leaves grow straight out of the sides of these stems and branches almost like an after thought, without any sheath formation. The leaf sheath anatomy allows one to 'prune' many monocots uniquely by just pulling off the oldest, lowest leaves by unsheathing them from the stem, leaving a clear, clean stem behind (not all monocots will unsheath like this, so don't be terribly surprised when you try to rip one off and it doesn't come off easily).
Close up of Aloe confusa, a vining, cliff-hanging species in my yard, showing the details of the leaf bases and how they sheath around the stem (left); Beaucarnea guatamalensis got fried by a freak freeze, and I had to cut all it's leaves off, exposing their leaf bases and layered sheaths (right)
Palms are monocots as evidenced by their leaf attachments to their trunks. Left is a rare Lepidorrhachis palm, one of many species with a crownshaft, which is a prominent leaf sheath that simply screams monocot (if you know what to listen for that is); right is a species of palm (Washingtonia) that does not form a crownshaft, but one can still easily see the splitting leaf sheaths covering the trunk of the palm
During a trip to the back yard I was able to tell many monocots by their sheathing leaves (Cordyline left and Dragon Tree (Dracaena draco) right)
Pulling off an Aloe barberae leaf, one can appreciate the sheath that attached it to the trunk of this large tree aloe (left); right shows a pile of leaf sheaths that have fallen off an Aloe pillansii (another tree aloe species that shed its leaves). Not all Aloes readily shed their leaves, but their attachments are basically the same
But these plants had leaves that did not attach by a sheath. Left is a common fruiting fig (thanks to the squirrels who transplanted this one themselves); and right is an Aeonium that at first glance looks like a monocot, but whos leaves leave ovoid scars where the leaves attach in an 'un-sheathlike' fashion
There are some group of plants in my yard that did not seem to fit the monocot symmetry. Some of these still have strap leaves that unsheath, but some do not. These include the orchids and the aroids (things like Alocasias and Philodendrons). Orchids just seem to be all about the flower, which is not something I am into, and aroids have leaves without parallel veins (always exceptions I guess). Though aroids have more complex "atypical" monocot leaves, they still tend to have a sheath attaching the leaf to the plant- there are almost always clues that can allow you to figure out if a plant is a monocot or dicot, even if they have characteristics that are not typical.
Though not able to keep either of these alive for more than a few years, these tropical monocots look a lot less 'monocotty' to me (Alocasia amazonica left and a Cymbidium orchid right)
While monocots tend to have strappy leaves with linear patterns or striations, dicots, by comparison, never have leaves with parallel veins (no exceptions?). They usually don't form rosettes, either, though it turns out there are some that do, too (see below). Dicot leaves usually have a ‘netted' vein pattern (if they have one at all), with a central mid-rib and little veins that head to the sides (think of a maple leaf- a typical dicot leaf!). And their leaves usually form much more randomly all over the plant, not from a central apical meristem (again, many exceptions found in my garden), but lots of little leaf meristems on every little branch (again obvious exceptions).
Some typical dicot leaves above (croton upper left, Japanese Maple upper right, Ficus petiolaris lower left and Magnolia delavayi lower right)
Above are some exceptions to the strap-leaf rule for monocots.. These are all monocots, but have more 'dicot-like' leaves with mid-ribs and veins that extend laterally. Still, all have other characteristics that make them monocots (other than just having a single 'cot'); top left is Alocasia cuprea 'green'; top right is the common decorator anthurium, Anthurium amnicola; next row left is Anthurium 'Obake', right is a lace-leaf anthurium, Anthurium podophyllum; 3rd row left is a common Philodendron solleum, right is a variegated Shell Ginger; and bottom row left is Monstera deliciosa and right is a Bat Plant (Tacca sp.).
What is the significance, if any, to the way these two plant groups form their leaves? Other than helping to identify which is which, there may be little of importance in terms of survival. But if you consider where the leaves come from- the apical meristems, their locations are of some importance. For example, if you cut a dicot's meristems, you will usually get brancing at that point as new branches and leaves will form in response (like what happens when you prune a rose bush). But the poor monocot, having only one central apical meristem, often will simply respond to its being lopped off by dying. This is why you can't hack off the top of a palm tree as it begins to threaten overhead power lines like you can dicotyldenous trees. I mean, you can, but you will end up with a dead post instead of a shorter palm tree. Some monocots tolerate this sort of 'hacking' of thier apical meristems, however (grasses and bamboo are just a few better known examples).
Poor Washingtonia trimmed by a stupid gardener who thought it would either branch or regrow after having its apical meristem removed (left). However, in younger palms, if topped but without completely removing the apical meristem (which is usually buried down a foot or so amongst the surrounding leaf bases), it can potentially recover. This recovery is not something one can depend upon, though (right photo).
Another lesson in Monocot versus dicot decapitation happened to in my garden just this year when an out-of-control motorcycle wiped out a lot of plants in the front yard. Left is a decapitated Agave sisalana, which is a monocot and did not recover (though it is a suckering agave and did leave a lot of progeny behind in that way); right is the decapitated Pachypodium lamerei one can glimpse as a white circle in the left photo at the far right corner. However, this plant is a dicot, and it is growing back as expected (photo on right is 6 months later).
Secondary growth is an important dicot characteristic. Branching is not unique to dicots, but monocots and dicots seem to branch in different ways. Monocots tend to branch dichotomously, which means the growth center divides and now 'twins' are born, and they continue on growing as paired stems or trunks. Dicots, having the ability to grow secondarily, often shoot out a branch wherever they "feel" like it (or so it seems). In other words, the branches to not have to originate from the primary apical meristem(s), but can grow secondarily.
Almost all dicot trees branch (left) and seem to do so almost randomly; right is a relatively rare, large, branching monocot, a Pandanus utilis. Note the somewhat symetrical way it was divided at each point, as well as the relative lack of diameter variation in trunk and branches themselves (remember, no true secondary growth in these monocots).
There are of course a few exceptions to this "rarely branching rule." Above are two Dracaena dracos (Dragon trees) that have branched considerably over the years, showing a widening trunk (secondary growth??) and numerous areas of branching (though still mostly dichotomously). At the tip of each branch is the rosette of strap-like leaves, all with their sheaths, and with a single apical meristem hidden in their centers.
Most palms do not branch, but this Hyphaene thebaica is an exception. Branching though looks different than it normally does in a large dicot tree
Secondary growth is also what allows dicots to form an ever-widening stem or trunk. Monocot stems grow to a certain size initially but then that's it. For exmple, have you ever noticed that palms, though often very tall, have a skinny trunk the same diameter the whole way up? Even the term woody stem is usually reserved for dicots. Palms and a few other big monocots have ‘woody-like' stems but these stems never grow thicker or have an ability to heal from injury once the plant begins to mature. The basic makeup of the stems differs, too, between monocots and dicots. All the vascular pathways (stuff like phloem and xylem- the cool words I sort of remember from school) are found neatly arranged around the edges of the stem or trunk in dicots (as the plant or tree grows a new ring forms with these structures in the outer ring) but randomly spread throughout the whole stem/trunk in monocots. So I guess ‘true wood' has rings and ‘false wood' doesn't. Again, why is this important? Well, if you hike up a palm tree trunk using spikes as you would a regular dicot tree, you will leave holes behind. But unlike a dicot tree, these holes will be unable to heal as there is no secondary growth to fill in damages like this. Same if you nail signs in palm trunks- those holes will stay there forever, allowing bacteria, fungi and bugs deep into the trunk. So, treat your monocotyledonous trunks and stems with a little more respect as they can't heal like a dicot trunk/stem can.
typical tree cut showing rings of growth (left). In this tree, all the important transporation units are located near the exterior of the trunk; right is a palm trunk cut (and a bit rotted) showing there are no rings at all. In these plants, the transporation units are pretty randomly spread throughout the trunk from its center to the edges. Smaller dicots and monocots with little stems are similarly arranged.
While very tall, old trees may produce a lot of secondary growth in the form of trunk diameter and butress roots (left), palms are left with few options but to just deal with their extreme height and non-widening trunks (see the amazing tall Livistona rotundifolia palm on the right). Fortunately, their trunks tend to be very flexible and rarely snap, even in gail force winds (just see what is on the ground after the next huge wind storm in your area if you live where there are palms- very rarely will one find fallen palms, but one always sees branches or entire dicot trees and conifers knocked over or snapped in half)... so there are a few advantages to the way their skinny trunks are made.
There is a big advantage to having a monocot stem and that has to do with the random placement of all the important transport mechanisms in the trunk. If you "girdle" a dicot tree (i.e., cut into it 360 degrees around and deep enough to cut into its xylem and phloem) it will die. Most of the time girdling is done on purpose to kill a large dicot tree, but sometimes gardeners will accidentally girdle smaller trees with their weed whackers and kill off their saplings. Monocot trees, on the other hand, can tolerate girdling since their transportion anatomy is not so easily cut off by girdling. They still might suffer an infection from such an event, but cutting into the trunk 360 degrees will not kill them outright as it will a dicot tree. So having a primitive trunk can have its advantages, too.
Monocots and dicots differ in their roots, too. As a dicot begins to grow, it sends down a single root (radical) called a tap root. This central root then puts out little side roots. And then these side roots put out smaller side roots etc. etc. So like the branches above ground, dicots also have a complex root structure below ground as well. Monocots, on the other hand, in their more typically simplistic way, just shoot out a bunch of basically branchless roots from the growing seedling called adventitious roots. Why is THIS important? This can make monocots a lot easier to dig up, a lot less destructive when it comes to structures below and around these plants, and a lot less finicky about being left in pots for too long. This last point is good for me as it lets me be lazier with my monocots and I can often get away with them becoming relatively root bound with seeming little consequences. Root binding is NOT something you want a dicot to have to live through, though (again, exceptions abound... just generalizations here!). And as I said, is it's harder to dig up dicot weeds (in general) than monocot weeds as dicot weeds have that long, deep, annoying and often fragile tap root (all these root qualities are essentially ‘points' for my team, the monocots!). Also, as just mentioned, large monocot trees (palms, aloes, agaves etc.) are far less damaging to their nearby environment than similarly sized dicot plants, thanks to their relatively wimpy adventitious roots. So you can plant these larger plants near homes, sidewalks and pipes without too much concern for future problems (at least from their roots). Note, however, that monocot plants that make rhizomes (bamboo, some palms, some grasses etc.), which are specially adapted root-like structures, CAN do a signficant amount of damage to nearby roads, sidewalks, pipes and buildings or be nearly impossible to dig up. Rhizomes are not true roots, though.
Bulbs are plants that never really left home. Instead they just developed a giant home base (takes over from the seed) that never goes away. Anyway, these are all monocots, too. Never seen a bulb with a tap root, have you? All have that typical hair-like mess of adventitious roots typical of the monocot society. True bulbs have layers of plant material in them (like an onion). Many of my favorite caudiciform plants have bulb-like woody, underground structures called tubers- these are NOT true bulbs, however, and many of these plants are dicots. Corms are not bulbs, either, but it seems corms belong primarily to the monocot group (Gladiolas, Dioscoreas, Beaucarneas etc.). Getting confusing, huh? Keep looking for tap roots and that should keep you straight on which are monocots or dicots, no matter what other root-like structures you find. And if that does seem to be there case, there is always some clue to let you know if the plant is a monocot or not.
Some of my favorite succulents are these 'above ground' bulbs, Pregnant Onion (aka Ornithogalum sp.) on left, and Climbing Onion (Bowiea volubilis) right
Two bulbs I have showing up periodically in my yard- Iris and Hyacinths. Leaves are strap-like and flower petals in 3s are also a monocot give-away
Flowers tend (and this is definitely not a hard and fast rule) to be trimeric (have petals in multiples of three) in the monocots while more likely to be in groups of four to five in dicots. There is a huge group of dicot plants, however, distinguished by their having trimeric flowers (that whole magnolia family and all their relatives, for example). Though most of the plants I think are so cool don't have too much going on in the flowering department, some of the most spectacular flowers on the planet belong to the monocots (orchids for example are monocots, as are daylilies, irises, tulips etc.). And some aloes have pretty amazing flowers, too (these are basically the only sources of seasonal color in my yard... thank goodness for aloes!). Figures the only dicots I really love, the Euphorbias, have worthless flowers for the most part. Even these Euphorbias, which have all their initial reproductive structures in groups of three, end up having four to five flower parts (or multiples of these).
The above Dyckia is a bromeliad with long, thin, radially symmetrical leaves, so one already might suspect it is a monocot. But after seeing its flowers (right), one can confirm that it is (note there are three petals per flower)
Two more terrestrial bromeliads, Puya alpestris (left) and Puya venusta (right). Note both have 3 petals per flower
though I know these are both bulbs, I can confirm their 'monocotness' by noting flower petals in groups of three (Scadoxis puniceus left and Scillia sp. right)
The Yucca flower on the right does not go with the Yucca desmetiana on the left, but nearly all Yucca flowers look the same to me. Anyway, in case one was in doubt of a Yucca being a monocot, the 3 petals might help one decide
With a plant I had never heard of before, I tried my monocot identifiers on this Anguloa plant, and sure enough, the linear striations on the strap-like leaves and the 3 petals made me guess Monocot, and I was right.
It was good to see that despite the Euphorbia's unique floral parts, most if not all Euphorbias failed to have 3 flower divisions (except for the female flower's stigmas were in 3s)- most of the rest had 1,2, 4 or 5 nectar glands, catophylls and bracteoles.
Just two of a gazillion dicot plants out there, showing typical petal counts of 5 (Hibiscus) and 4 (Begonia).
Hydrangeas on left may look like complex flowers, but really these are just bunches of little flowers with 4 petals each. Kalanchoe marmorata, a common succulent dicot, also has 4 petals per flower (most succulent dicots have 5 though).
This, for me, is where this floral thing breaks down- where the flowers are a bit too complicated for me to try to decide what's a petal, what's a carpal, what's a sepal etc. The Leucospermum on the right is a dicot and the Aehmea on the left is a monocot (Bromeliad)... but I would have trouble confirming that from the flowers alone on these ones.
The Anigozanthos on the left is an obvious monocot based on the strap leaves, but the flowers are bizarre and hard for me to tell what is a petal... this is the best shot I could come up with that maybe shows 3 petals; The Bougainvillea on the right may look like it has 3 petals, but those brilliant colored parts are not flower petals. The itty bitty white thing in the center, with 5 petals, is the flower (hence dicot)
The Xanthorrhoea on the left screams monocot by its overall shape and grassy appearance (though it is not related to grass); but if all I had was its flower on the right, I would be hard pressed to say which family it belonged to... not too many obvious petals. Turns out almost all plants with conical pannicles like these are monocots.
and these are even tougher for me... both are dicots (can see from the highly branched plants) but I can't make out any petal numbers (Melaleuca fulgens left, and Rosa 'Jasper' right)
Why is it important that monocots have sets of three petals while dicots have four to five? Good question. That's just they way it is. Sort of an evolutionary thing. Perhaps some reader can chime and tell us all what is practically important about the number of petals a plant has.
Last, and least, the pollen is different between dicots and monocots... in monocots, pollen grains have a single pore or groove, while dicot pollen grains have three. Big whoop. But I had to mention it since it is a scientifically ‘interesting' difference and seems to be a pretty consistent feature (for those that happen to have a microscope handy). Why is THAT important? It's not. Just a nerdy thing to note and recall I am sure (it probably has some evolutionary signficance, but it's too much minutia (literally) to discuss more here.
What about cacti? They seem somewhat primitive and simple- just a blob or column of fleshy tissue covered by spines. Well, it turns out, despite often having radial symmetry and rarely any branches (though some do), cacti have tap roots (at least they all start out that way... many cactus sure appear to have a bunch of adventitious roots to me, but I am not a botanist). And sure enough, as cactus seeds germinate, they send up those two little leaves. So Cacti are dicots, too. I have seen some cactus seed germinate and sure enough, they almost all start out with two dinky fleshy little leaves, even though many (most) will never form another leaf again their entire lives. Even the vast variety of living stones, the featureless bumps of rubbery tissue that often are no larger than a thumb, often lumped together in a group called the mesembs, are dicots. As dinky a roots as they have, they each have a tap root.
Cactus flowers tend to be pretty spectacular but the petal numbers are a bit staggering. I find that any flowers that have over 6 petals are probably dicots (Echinocerus triadigloditus left and Cereus aethiops right, from my garden)
Despite many cacti's radial symmetry, and primitive appearance, turns out they are dicots. Right photo are newly sprouted cactus seedlings showing their definitely paired new cotyledons (photo by CactusJordi). Unless one actually sees these, it is hard to imagine, but all cacti start out the these paired cotyledons, even though nearly all are leafless from then on (note: these are exceptionally prominent cotyledons. some cacti arise from the seed with a more typically globoid shape and only the hint of paired cotyledons on each side of the globe- see below)
Two more photos by CactusJordi showing newly germinated cacti. Plants on the right show some paired cotyledons, but those on the left look like globoid structures. However, they, too, have paired cotyledons, but they are very hard to see.
Another photo by CactusJordi. This is an Opuntioid species, one of the more primitive cacti... turns out the more primitive the cactus species, the larger the cotyledons (very easy to see in this photo!)
So, armed with all this new information, I wandered about the yard and local botanical garden seeing if I could use these differentiating characteristics to help tell which plants were monocots and which were dicots. For the most part, I knew the basic structural plans, so anything with a lot of branches was a dicot, all bulbs were monocots (though I personally don't always know what is a bulb and what is not without digging it up), grass, palms, bromeiads and all agave and aloe relatives are monocots... but when it comes down to secondary trunk growth, flower details, tap roots, pollen grains etc. I really did not much more of a clue than that. Most flowers it turns out are way too complex for me to see if they have multiples of 3 or more... many flowers did not seem to have petals at all. And too many plants have huge, caudex-like trunks even though I know they were probably still monocots. And I saw a lot of branching monocots as well. And there were a number of surprises, too.
As it turns out some of my plants I had assumed to be monocots all along, thanks to their radial symmetry and rather 'agave-like' growth patterns, (rosettes with one leaf at a time growing from the center) are indeed dicots. The Aeoniums, Echeverias and Dudleyas are the main plants I am thinking about now. I didn't even think about it until I happened to pick off an Echeveria flower today, while writing this article, and sure enough, five flower petals... oops. Not the three I was expecting. But then all one has to do is think back to what major plant group these belong to (the Crassulaceae), and realize they are closely related to Jade plants (Crassula ovata) and note those plants have lots of branches and secondary growth of their trunks. Probably have tap roots, too, though that's hard to tell with some plants. I then lookd closely at their leaves and noted that I could not identify vein structures in any of the leaves of my Aeoniums, Echeverias or Dudleyas. So they may not have netted veins, but they don't have parallel veins, either. Many have a central midrib they way a lot of dicots, do, though.
Aeoniums to me were a bit of a surprise, showing very typical radial symmetry and leaf growth of a monocot, and roots that did not appear to have much in the way of a central radical or tap root (right).
But their flower petal numbers confirmed these are NOT monocots (each of these species have 8 petals, not multiples of three). And a closer look at their leaves revealed no real striations or veins of any kind at all.
The Brighamia insignis on the left looks a bit like a palm, has a single trunk and leaves that radiate symmetrically from a central apical meristem. But the leaf scars are NOT typical of a monocot, and sure enough, this turned out to be a dicot. Right is a Cala lily in the garden, which did not have the flower petals in 3 that I could tell, nor leaves of the right shape, but since it was a Lily, I assumed it was a monocot, and I was right- no tap roots
Canna 'Tropicana' on left had too floppy a flower for me to tell if there were multiples of three petals, and the leaves have dicot-like markings... but I have dug these up before and they have corm-like root parts, so I still guessed monocot, and was right again. Right is a close up of Crassula rupestris showing 5 petals per flower, so another dicot despite its somewhat monocot-like simplicity
Dudleyas have a very aloe-like look to them, but their leaves, despite being lancelote are radiating from the center have no veins or striations at all, so they are not helpful as monocot vs dicocot determination. However, flowers show 5 petals (right... not the flowers of plant on left by the way, but another species of Dudleya so these are monocots.
I ran into a similar situation with the Echeverias, which also look monocot-like in shape. This Echeveria subrigida looks a lot like an Aloe or Agave in overall shape.
But the flowers of Echeveria subrigida (and all Echeverias by the way.. in fact, all Crassulaceae except the Kalanchoes) have 5 petals (left). Another Aloe-like Echeveria on right, Echeveria metallica.
This Echium wildprettii has some monocot-like leaf shapes and radial symmetry (left), but the huge flower pannicle it cranks out after 2-3 years shows 5 petals per little flower- another Dicot.
Strelitzias, or Bird of Paradise, have pretty exotic, weird flowers that I find hard to pin down whether there are 3 petals or not. Leaves are distinctly dicot-like, and the roots of this plant are very thick, though I could not discern a tap root. But, when one rips out a dead leaf from one side of the plant, one can see the leaf sheath characteristic which gives away it's 'monocotness'.
Of course, as mentioned above, not all plants fit into these two categories... all flowering plants do, though. Turns out some of my favorite plants are in a completely separate group of plants- the Gymnosperms (including the pine trees and cycads). So some plants I have LOOK like they might be monocots, due to their radial symmetry and all (the cycads), but their not making any flowers (cones instead) keeps them out of that club. And the ferns, another group of plants that grow with some radial symmetry sometimes are not even gymnosperms and don't even make seeds (spores instead). More on these groups later (another future 'intellectual' article).
So now when you hear a nerdy discussion about a plant being referred to as a monocot or a dicot you'll know what they are talking about and can even sound all scientific and add to the conversation! And you can start looking for some of these differences and see which plants are your favorites.