The Carrot Family - Apiaceae

Repetition is the essence of the plant

Theophrastus 372-287BC

This post will review a family, several members of which will be putting in an appearance in the park over the next few weeks. The Apiaceae is one of the most distinctive of all plant families and its members were the first to be lumped together into a distinct grouping as long ago as the end of the 16th century. This was on account of the shape of the inflorescence called an umbel which in turn gave rise to the old family name the Umbelliferae. That family name has gone, at least in part, because some flowers from other families also have umbels.
The members of the Apiaceae that we will discuss over the coming weeks are cow parsley Anthriscus sylvestris, hogweed Heracleum sphondylium and finally my favourite wild carrot Daucus carota.

Habit

Members of this family often have stout stems that are ridged and hollow, a recipe that gives them the strength and rigidity that can allow members of this family to grow to great heights seemingly out of proportion to the diameter of their stems. For example, giant hogweed can grow up to 3 metres in height and anyone looking at at the tall slim stems of cow-parsley will be amazed that it can remain upright in anything more than the gentlest breeze. Nonetheless, other members of this family can be quite delicate.

Leaves

The leaves are usually pinnate and just how pinnate they are can be a diagnostic feature. Pinnation requires some explanation. A simple leaf has a single blade although that blade can be deeply lobed or toothed. A compound leaf, however, is composed of multiple discrete leaflets. Pinnate leaves can be thought of as a subset of compound leaves in which the leaflets are arranged on either side of a central axis called the rhachis. In the case of one pinnate leaves, there is a row of single leaflets on either side of the central rhachis. In the case of bipinnate (2-pinnate) leaves, the leaflets are born upon second-order branchings (see pictures 1and 2) and in the case of 3-pinnate leaves, the leaflets are born on third-order branching (see picture 3). The leaves can appear so divided that it is possible at a casual glance to confuse them with ferns (I have embarrassed myself in this way). However, if you look at the petioles of the Apiaceae, they are deeply V or U shaped as they join the main stem. 

1. Schematic showing a simple leaf, a 1-pinnate and a 2-pinnate leaf.

2.  Cow parsley. Showing 2-pinnate leaves. The axis of the first order branching in yellow
and the second in red. 

3. Hemlock. Showing 3-pinnate leaves. First branching from the main stem in stem in yellow,
second in red and third in blue. The leaflets are borne on the 3rd order branching.

Inflorescence

A key feature of this family is the umbel which consists of several rays emanating from the central stem. The flowers may be arranged on the end of each ray but more commonly, secondary rays emanate from the primary ray to form secondary umbels upon which the flowers are borne.
The umbel may be subtended by a bract and the secondary umbel by bracteoles. The presence or absence of these bracts and bracteoles can be an important diagnostic feature if you choose to use identification keys. The usual end result of this arrangement or rays and secondary rays is a flat-topped inflorescence of numerous small flowers that is a haven for all kinds of pollinators. No specialist equipment is required to sample the nectar and for that reason, this family is often favored by beetles, hoverflies and other insects that do not have a long proboscis.

4. Schematic of an umbel of umbels

5. Cow parsley shot from below. Note rays emanating from the main stem and secondary rays 
emanating from those rays. Secondary rays are subtended by bracteoles.

6. Secondary umbel shot from below. Note bracteoles subtending secondary rays.

Flowers

The flowers are characterised by an inferior two-chambered ovary that you can see in the picture above immediately below the petals (6). You can feel the inferior ovary if you gently place two fingers together under the plane of the petals of an individual flower. The two styles that can usually be seen protruding from the ovary often persist when the petals have dropped off and the fruit is developing. The calyx may be represented by tiny teeth or be absent altogether. There are 5 free petals and there tends to be a degree of asymmetry. The outermost petals are longer than those facing the centre of an umbel or secondary umbel and those flowers at the edge of an umbel are more asymmetric than those at the centre (picture 8). There are five stamens that are found alternating with the petals.

7. Left. Schematic of a flower showing 5 free petals alternating with stamens.

8. Cow parsley - individual flowers showing five petals and two protruding styles.

Fruits

The fruits of this family are often spectacular and an important diagnostic feature. They will be discussed in more detail when individual species are discussed in future blogs. Like so many other flowers it is worth returning to them on your park walks after the flowers have 'gone over'.