The square stem is common to all members of the mint family (Labiatae, Lamiaceae). They have four vascular bundles arranged in a square cross sectional pattern, which is always present. There are some exceptions, but even these show the four-fold pattern at early stages. Occasionally, Salvias will send out a side shoot with six-fold symmetry that will be maintained through its growth, but not with its side shoots. The leaves are always opposite, in whorls of 2, occasionally 3, and even 4 in Westringia.

Westringia (Australian Rosemary Bush) has eight fold symmetry, but juvenile growth from seed advances from four through six to eight leaves, bundles, and nodes per axil.

As the stems thicken from growth and form bark, the square exterior can often be submerged in the bark.

Other families, including the daisy family (Asteraceae) have some genera that favor square stems. In others, polygonal stems are found, such as in the Rubiaceae (bedstraw, sweet woodruff)and Acanthaceae. This does not guarantee that leaves will come out in whorls or in pairs (opposite).

There is one Salvia which does not have square stems, it is S. wagneriana, which is sometimes called S. involucrata ssp. Deschampsiana in Europe.

Robin.

Epling's section Cardinales includes wagneriana, karwinskii, puberula, involucrata, holwayi, and stolonifera. They all show this aspect, more or less. I think an examination of the cross section will still show the four vascular bundles.

The variations have evolved to allow adaptation to local environments. Salvia madrensis probably has strongly angled stems to confer wind resistance. This may reflect that they are stand alone or taller than other plants in their community. Their nodes are also more resistant to snapping. Section Cardinales stems look like they were meant to be flexible and may compete with nearer neighbors of the same or similar height.

There is a definite correlation to all the structural features of Salvias and their position in the environment. Part of my enjoyment of growing them is letting my imagination try to envision what their environment is like. It is even more fun to go into the field and challenge these ideas by observing them. Afterwards, this information can be applied to their horticulture.