This is an illustrated key to the families of Gymnosperms. These are woody plants, exclusively wind-pollinated, of very ancient origin, including many extinct families and genera. 
The key feature is that the ovules are naked (left) not enclosed inside an ovary as in the Angiosperms (right). The ovules, and subsequently the seeds, are typically borne on the face of a cone scale. The pollen enters the micropyle directly, not via a pollen tube down the style
Palm-like or fern-like plants with a trunk less than 2m tall, composed of leaf bases supporting a rosette of large, compound leaves. These belong to the order Cycadales.
The order Cycadales comprises the families Cycadacea (e.g. the familiar Cycas revoluta; left), Zamiaceae (8 genera including Encephalartos; right) and Stangeriaceae (just Stangeria and Bowenia). Otherwise, next tweet.
Small shrubs of a switch-like habit are the order Gnetales, which contains some famous plants including Welwitschia (left; Welwitschiaceae), and Ephedra (right; Ephedraceae). Otherwise, next tweet.
Leaves broad (>2cm), fan-shaped, with a long petiole, deciduous (left). This is Ginkgoaceae, represented by the common urban street tree Ginkgo biloba. The buds on the winter twigs are highly distinctive (right).
Ginkgo trees are male (left) or female (right) i.e. dioecious. Males are typically grown because the fruits borne by females are unpleasantly smelly ("a nauseous stink" according to Stace). Plant not like this, next tweet.
Leaves stalked but narrowly linear lanceolate (i.e. not broad) up to 20cm x 3cm, with seeds typically inside a fleshy aril are Podocarpaceae. There are 6 genera. The commonest species are illustrated in this and the next 2 tweets. Dacrydium (left), Lagarostrobos (right).
Podocarpaceae genera II: Microcachrys (left), Phyllocladus (right)
Podocarpaceae genera III: Podocarpus (left, now known as Prumnopitys andina), and Saxegothea (right). Plants not like this, next tweet.
Leaves sessile, broad, many-veined and leathery, closely spiralling the stem, and buds lacking proper bud scales, are Araucariaceae.
The Monkey-puzzle Araucaria araucana (last tweet) is the only common tree outdoors in Britain, but the tree you remember from your Mediterranean holidays, A. heterophylla (right) survives the winter outside in London these days.
Shrubs with lanceolate leaves in 2 lateral ranks, tapering gradually to a sharp tip. This family is Cephalotaxacea. There are just two species, both in the genus Cephalotaxus: left is C. harringtonia (short leaves < 6cm); right is C. fortunei (longer leaves, up to 9cm).
If your plant is a bit more Yew-like, and has parallel-sided leaves with rounded, obtuse tips, and buds with green bud-scales, then look for fruits. If you find solitary seeds more or less enclosed in a red fleshy aril (i.e. a bit berry-like) then your plant is Taxaceae.
Our native is Taxus baccata, of which there are dozens of cultivars to be seen on any urban walk ('Fastigiata Aurea'; left)' 'Fastigiata'; right). There are 4 other species and 2 hybrids grown in British gardens, all of which are rare and well worth seeking out.
Finally, we come to the big two Gymnosperm families: Pinaceae and Cupressaceae. The Pinaceae have needles for leaves, either singly or in bundles. There are two sorts of Cupressaceae: redwoods (formerly Taxodiaceae) with awl-shaped leaves and cypresses with scale-like leaves.
These groups will get close attention both when we look at how to tell the genera apart (next thread), and then even more detail when we look at keys to the species within each of the genera. For now, just enjoy the families.
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