Wednesday, 22 March 2017

Forsythia x intermedia - introducing Oleaceae (the olive family)

The yellow flowers of Forsythia are a familiar sight in gardens in the spring.  The garden form is a result of crossing the Japanese species Forsythia suspensa and Forsythia viridissima repeatedly over the last 120 years or so.  Hybrids are given the genus name followed by a letter x to represent it is a cross between two or more species and are given a hybrid name. 

Forsythia is named after William Forsyth, a Scottish botanist who managed the gardens at Kensington and St James’, London from 1779.

The flower has four green sepals which are joined towards the base, forming a cup.  These make up the calyx which is said to be gamosepalous as the sepals are joined.  The corolla is bright yellow and is divided beyond half way into four lobes. The lobes of the corolla are joined at the base so the flower is called gamopetalous. The united part of the corolla forms a cup-shaped structure.  The veins running into the corolla lobes are coloured towards the base. 

In the centre of the flower are two short stamens.  The filaments are attached to the corolla near the base, in line with a cleft between two of the lobes - they are termed epipetalous. The filaments are attached to the base of the anthers - they are basifixed.  The anthers open to expose the pollen outwards (extrorse).

There is a single ovary which sits adjacent to the base of the filaments and the corolla, so the ovary is superior and the flower is hypogynous. Attached to the top of the ovary is a single style.  At the top of the style is a stigma which is divided into two rounded lobes.  If the ovary is cut across with a blade it is found to be formed of two hollows (bilocular) each containing a number of ovules which are attached to the top of the ovary - apical pendulous ovules.  

Also in family Oleaceae in Britain are the native ash tree (Fraxinus) and privet (Ligustrum) along with cultivated species of privet, jasmine (Jasminum) and lilac (Syringa).  All of these have only two stamens, four sepals, fused towards the base and leaves that are opposite one another.

Introducing the floral formula

The floral formula is a shorthand way of indicating the structure of a flower using simple notation.  

In the formula above for Oleaceae, the sex sign on the left indicates that the flowers have both male and female parts.  Ca with an encircled 4 shows the calyx is made of 4 lobes, joined at the base.  Co with an encircled 4 shows the corolla is also made of 4 lobes joined at the base.  The G with an encircled two shows the gynoecium is formed of two locules that are joined together into a single structure with one style and stigma.  The A with the two shows there are two anthers that are separate from one another.  This is underlined and placed above the corolla showing they are attached to the corolla.  Finally the calyx, corolla and gynoecium are all underlined showing they are all attached at the same level, indicating the ovary is superior. 

In botanical speak this would be written: Flowers hermaphrodite.  Calyx gamosepalous, four-lobed.  Corolla gamopetalous, four-lobed.  Anthers two, epipetalous.  Gynoecium with two united carpels, superior.

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