How are angiosperm orders defined (for the layman)?

Most of the orders of land animals make sense to the average layman. Lepidoptera are butterflies and moths; Diptera are flies; Carnivora are, well, carnivores; Artiodactyls are cloven-hooved grazers and browsers; Rodents are obviously rodents; Anseriformes are ducks, geese, and swans; Ciconiiformes are herons, ibises, storks, and other longlegged wading birds; and so on. Some of the bird and fish orders take a little getting used to, but they do make sense after you catch the distinguishing features.

However, the ideas underlying orders of flowering plants are no doubt founded in some sensible taxonomy: anatomy, primitive shared characters, and the like. But the orders don’t make sense: mixtures of trees, shrubs, and herbs that don’t have obvious common characters. Does any botanist have some simple, easily grasped rules for distinguishing angiosperm orders?

Generally, botanical classification is done on the basis of characters that aren’t easily apparent to the average layman. The most important are usually floral characteristics: the numbers of stamens, pistils, petals and sepals; how the compartments in the ovary are arranged; how the ovules are attached within the compartments; etc. Traditional botanical keys are often of little use when flowers are not available, and even when they are they may require the dissection of tiny floral part. In the tropics, where family diversity is very high, many botanists will refuse to even try to identify material unless flowers are present.

Modern botanical classification has if anything made it a bit harder. Traditionally, closely related herbaceous and woody groups were often separated in different families, but today the tendency is to lump them.

Plant classification is largely about SEX. Yes, SEX! Flower parts (a plant’s genitalia) are the major identifiers. Blame Linnaeus, that repressed Swede…

To expand a little more, as someone who is not a botanist per se, but has spent a lot of time identifying plants in the field, I couldn’t tell you the characteristics of most orders (and I’ll bet a lot of botanists couldn’t, either). Families are a lot easier to recognize in the field, especially when it comes to non-flower characteristics. Some can be recognized, for example, by characteristic odor of sap or color of latex; the type of hairs on the leaf; venation patterns of leaves; or structure of wood. But often such characteristics have exceptions, so that it can be impossible to identify all members of a family by sterile characters alone.

Plant orders are very technical, because as others have said, differentiation in plants has to do with primarily reproductive structures and arrangements, also stem, leaf, and root anatomy. Here’s a few to give you an example:

  • ACORALES (acorus)

Inflorescence a spadix [dense spike] with spathe; flowers weakly monosymmetric, tapetal cells 2-4-nucleate, pollen endexine spongy, carpels ascidiate-plicate, ovules atropous; endosperm cellular [first division transverse, divisions in each domain similar], copious; collar rhizoids +.

  • POALES (Grasses and grass-like plants)

Mycorrhizae absent; vessels also in stem and leaf; SiO2 epidermal; raphides 0; septal nectaries 0, styles separate, or single and strongly branched, dry; endosperm nuclear, embryo broad, short to minute; mitochondrial sdh3 gene lost.

  • ARECALES (Palms and close relatives)

Plant woody, usu. monopodial; vessels also in stem and leaf; cuticular waxes as aggregated rodlets, stomata tetracytic; leaves spiral, petiolate, plicate, pinnately (palmately) pseudocompound or deeply divided; septal nectaries +, 1 apotropous ovule/carpel; embryo short, broad.

  • ROSALES (Roses and close relatives)

Dihydroflavonols +; roots diarch [lateral roots 4-ranked]; prismatic crystals in ray cells [not Barbeyaceae, Elaeagnaceae]; (sieve tubes with non-dispersive protein bodies; sieve tube plastids lacking starch [Rhamnaceae, Dirachmaceae?]); mucilage cells +; stomata anomocytic; leaf margins with teeth; inflorescence cymose; hypanthium +, nectariferous, K valvate, C clawed, 1 apotropous ovule/carpel, micropyle endostomal, styles +, stigma dry; K and/or hypanthium persistent in fruit; (polyembryony +).

Within plants, appearance is so diverse that you can’t necessarily give general appearance without being specific, because many plants mimic others.

Just so it doesn’t appear I misunderstand you, I did, My examples are to illustrate why plant orders tend to be very specific and technical.