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Many plants of the mint family have square cross-sectional stem shapes, but this characteristic is not confined exclusively to the mint family. Some Willowherbs, some Figworts, some Bedstraws, some Plantains, some Saxifrages and some St Johns Wort plants also have square stems.
Far fewer plants have triangular cross sections. There are some plants which seem to have 5, 6 or maybe even more sides, and a Broom with ten facets. Some Sow-thistles have multi-faceted stems especially in those parts lower down nearer the roots.
Most leaves, of course, have only two sides.
Some stems are solid, whereas others are hollow. Some have ridges (which could vary from two to four in number), or even have wings up the stem (a bit like moulding flashes). Those with wings may have two opposite, or four at right-angles.
  SQUARE
These stems are square in cross-section, and may be ridged, furrowed or hollow as well.
 ANGULAR
These stems are angular in cross-section with more than four faces, and may also be ridged, furrowed or hollow as well.
 OVAL
These stems are oval in cross-section rather than round, and may also be ridged, furrowed or hollow as well.
 LENS
These stems are lenticular or lens-shaped in cross-section rather than round, and may also be toothed on the edges or ridged as well.
 TRIANGULAR
These stems are tri-angular in cross-section with only three faces, and may also be ridged, furrowed or hollow as well. The actual shape may be isosceles or equilateral.
 ROUND
These stems are round in cross-section and may also be ridged, furrowed or hollow as well.
 WINGED
These stems have wings or 'flanges' on the stems, usually running parallel to the stem. They may have two, or more wings. The stem itself could be any shape, round, square, whatever. For the purpose of this website only, 'winged' also includes those plants with 'welted' stems, such as on Welted Thistle , where the wing may be dis-continuous, or spiral around the stem.
 RIBBED
These stems have ribbed or ridged stems, but the stems themselves could be any shape, not necessarily square as shown.
 FLUTED
These stems have fluted or furrowed or perhaps slightly concave stems, but the stems themselves could be any shape, not necessarily squarish as shown. Examples are Bilberry, Rhubarb (although these are the stem leaves, which are extremely concave, like celery) and Broom, although one mans flute or furrow looks like another mans rib or ridge...
 HOLLOW
These stems are hollow in cross-section. They may be round, square, triangular or any other cross-section and may, or may not be, full of a milky latex or sap.
 SPINES
These stems have sharp spines or thorns or perhaps stiff bristles on the stems and/or leaves. Enough to scratch or prick the skin.
Sharp objects of the stem which can penetrate the skin come under at least three botanical names (which are not interchangeable):
- THORNS are modified branches and usually occur near the end of a main branch. Examples of plants with thorns include
Sea Buckthorn , Buckthorn , Hawthorn and Blackthorn (Sloe).
- SPINES are modified leaves or stipules and usually are found directly under a leaf-scar or bud.
Examples include Barberries (species of Berberis), species of Robinia such as
False Acacia , species of Ribes such as Gooseberry or Blackcurrent .
- PRICKLES are extensions of the epidermis (the outer layer of twigs) and can occur anywhere between nodes on twigs. Examples include species of Rubus (Brambles) and species of Rosa (Rose), on the stems of
Giant Rhubarb s, Small Teasel , Prickly Lettuce (on midrib of leaves), Bristly Oxtongue (leaves and stem), Thorn Apple (on fruit) and Thistles (on leaves, stems, flowerheads). None of these species have 'thorns' although they are commonly so described by laymen (including your Author...)
Your Author is not sure where Stinging Nettle trichomes (which are hollow hairs filled with noxious substances) fits into this botanical nomenclature scheme for spines but they certainly penetrate the skin easily and hurt a lot more than these other more substantial spines.
 MILKY SAP
orange
These stems, when broken or cut, ooze an (often toxic) milky sap latex. This may be milky white, yellow, orange, red or, less often, some other colour such as cream or green. If the latex is not white then underneath the icon will be a word saying which colour the sap is, as here (orange).
Plants yielding a white latex include many (all?) plants from the families: Asteraceae , the Lactucaceae tribe from Asteraceae , Campanulaceae (including some green or cream latex ones shown below), Convolvulaceae and Papaveraceae (where the latex often changes to yellow or red on drying) and only from some plants in the Aceraceae , Alismataceae and Apiaceae generas plus species of Ficus , Morus , Rhus and Vinca .
Plants yielding yellow, orange or red latex are from the Scorzonera family but only some from Papaveraceae families. The latex is confined to the roots in Eschscholzia genus plants.
Those yielding a cream or green latex are just single species: Wild Angelica (Angelica sylvestris ), Broad-leaved Harebell (Campanula rhomboidalis ), Nettle-leaved Bellflower (Campanula trachelium ), Masterwort (Imperatoria ostruthium ) and Spiked Rampion (Phyteuma spicatum ).
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