Apocynaceae family.
The Oleander family can be defined in a loose or strict sense (s.l. or s.s.).
These differences in classification result in widely varying figures.
The number of genera range from 164 to 450 and species from 1500 to 5100.
Apocynaceae (Dogbane family) and Asclepiadaceae (Milkweed family) were originally separate families.
Most now reduce them to subfamilies in a new, more loosely defined Family Apocynaceae s.l.
A common classification is into 5 subfamilies.
Apocynoideae and Rauvolfioideae are part of the original Apocynaceae sensu stricto.
Asclepiadoideae, Periplocoideae and Secamonoideae were part of family Asclepiadaceae s.s.
They are found worldwide in the tropics and subtropics.
Genera include Adenium, Allamanda, Beaumontia, Carissa, Catharanthus, Hoya, Mandevilla, Nerium,
Pachypodium, Parsonsia, Plumeria, Stephanotis, Tabernaemontana, Thevetia, Vinca and Wrightia.
Alstonia, Alyxia and Ochrosia are from northern Australia.
Most are lianas (woody vines) with a few trees, shrubs, herbs and succulents.
The lianas are mostly stem twiners, sometimes with thorns.
The tissues have a milky latex but in a few the sap is clear, red or yellow.
Many plants are poisonous if swallowed.
Simple hairs, with or without glands, may be present.
The simple leaves are usually opposite and often decussate (alternate pairs at right angles).
Others are in whorls of 3 to 7 and they are rarely alternate.
There are usually no stipules but occasionally there are small, stipule-like glands
or a raised line between the petiole bases.
Glands are occasionally found on the petiole or leaf blade.
Some genera have tufts of hairs on the leaves or tiny pits.
Inflorescences, axillary or terminal, are branched or unbranched clusters or occasionally a solitary flower.
There may or may not be bracts and bracteoles.
Flowers, on stalks, are bisexual and radially symmetric.
There are marked variations in flower structure within the broad family.
The perianth parts, especially in the bud, overlap and are twisted to the right or left.
The calyx has 5 (4) sepals usually with their bases fused but occasionally separate.
The 5 lobes may overlap (imbricate) or lie side by side (valvate).
The persistent sepals, often bent backwards, may have glands at their base inside.
The tubular, funnel or bell-shaped corolla is commonly twisted.
There is often a corona or a ring of small scales or hairs in the throat.
Colours include white, blue, pink, purple, red or yellow.
In Apocynoideae.
The 5 (4) stamens alternate with the corolla lobes and are attached to the tube.
They are free with short filaments and basifixed anthers that open inwards via long slits.
The pollen is shed as separate grains.
In Asclepiadoideae.
Genera such as Hoya, Stephanotis and Gomphocarpus have very different centres.
The stamen filaments, attached to the base of the corolla, are fused into a stout column.
From the sides of the column project 5 fleshy pouch-like lobes (hoods).
These ‘U’ shaped lobes are attached at their bases, folded vertically and open at the top.
There are one or more small beaks or horns on the upper, inner corners of each hood.
Inside the column are 2 separate ovaries and styles.
The 2 stigmas fuse into a large, flattish, 5-angled stigmatic head.
The anthers are fused to the sides of this forming a gynostegium.
Each anther has 2 pollen sacs or theca.
The pollen grains in each are fused into a solid pollinium.
The pollinia of adjacent anthers are connected by a translator apparatus.
This consists of 2 arms (the retinaculum) with a small gland (corpusculum) in the middle.
The translator apparatus is inside a thin sheath between the anthers that has a vertical slit.
The corpusculum protrudes through the slit.
The translator and the paired adjacent theca with their solid pollinia are shed as a unit.
In all subfamilies the mostly superior ovary has 2 (5 – 8) carpels.
Commonly the ovaries are free and the top of the styles may be free or fused.
There are 2 to many ovules in each locule.
There is often a nectary in the form of a disc or 5 glands at the base of the ovary.
The fruit are follicles that are often paired, or a berry, capsule or drupe.
The often flattened seeds may have one or two tufts of hair or a wing.
J.F.
