The saxifrage family
Saxifragaceae is the family of cold rock. It reaches further north than almost any other flowering plant, it lives in cracks where there is no soil to speak of, and its name is a two thousand year old claim about what it does to stone.
around 30 genera
roughly 600 species
Northern temperate, arctic and alpine, with very little in the tropics
Everything in this guide that belongs to Saxifragaceae
The name is Latin and blunt: saxum, a rock, and frangere, to break. These plants root in fissures in cliffs and boulders, wedge themselves in, and over time the crack widens, so it looks very much as though the plant is splitting the rock rather than merely occupying the split.
Several of them add to the impression. A whole group, the silver or encrusted saxifrages, pushes hard water out through pores along the leaf margins and leaves the dissolved lime behind as it evaporates, so every leaf is edged with a neat row of chalky white dots. A plant growing out of limestone and apparently producing limestone on its own leaves made the story easy to believe.
The name also carried a medical claim. Under the old idea that a plant's appearance advertises what it treats, something called stone breaker was an obvious candidate for kidney and bladder stones, and it was prescribed for exactly that for centuries on the strength of little more than the name.
Purple saxifrage is one of the northernmost flowering plants on the planet. It has been recorded above eighty three degrees north in northern Greenland, on ground that is bare gravel for most of the year, and it is typically the first thing to flower anywhere it grows, often coming into bloom within days of the snow going and sometimes through it.
The shape is doing the work. These plants grow as dense low cushions, which trap still air among the leaves and can hold the inside of the cushion several degrees warmer than the air a hand's width above it. In a place where the growing season is a few weeks, a few degrees is the difference between setting seed and not.
Some arctic members do not risk seed at all. Instead of flowers they produce small bulbils on the stem, ready made miniature plants that drop off and root, so reproduction does not depend on a pollinator arriving, on the weather holding, or on there being enough summer left to ripen anything.
The bishop's caps are worth finding a photograph of. Their flowers are small and green and look like nothing from a distance, but each petal is divided into a set of fine radiating filaments, so a single flower under a lens is a row of tiny white snowflakes.
What follows is better. The seed capsule opens into a shallow open cup with the seeds sitting loose in the bottom, and it stays that way, doing nothing, until it rains. A raindrop landing squarely in the cup splashes the seeds out and several inches clear of the parent.
It is a real dispersal mechanism with a real name, and it only works because the cup is the right width and depth for a falling drop. The plant has effectively subcontracted the job to the weather and pays nothing for it.
The fastest change in this family has happened in gardens rather than in the wild. Coral bells were, until fairly recently, a modest American woodland plant grown for thin spires of small flowers. Breeders then started crossing a handful of species and selecting for leaf color instead, and inside about thirty years the genus went from a few green-leaved kinds to hundreds of named cultivars in amber, lime, peach, near black and silver.
It is now grown almost entirely as a foliage plant, with the flowers treated as incidental and sometimes removed. Very few ornamental genera have been redirected that completely that recently.
For the wider neighborhood, this family sits in the same order as the peonies and the stonecrops, both of which look nothing like it, along with the currants and gooseberries and the witch hazels. It is one of the more mixed groups in the classification, and one of the older ones.