Purple Loosestrife

Lythrum salicaria

" I am stark staring mad over Lythrum. For the love of Heaven have look at some of your species and if you can get me some seed, do!"
Charles Darwin. Letter to Asa Gray

The reason for Charles Darwin's puzzlement will become apparent later but bear in mind that when he wrote this in a letter to the American botanist Asa Gray, he had just finished his book on Orchids so we can be sure he was looking at a deeply mysterious arrangement of flower parts.

Taxonomy

Purple loosestrife belongs to the Lythraceae family which contains 32 genera and 620 species. The type genus is Lythrum or the loosestrifes. The leaves are typically opposite, simple and with pinnate veins. The flowers are radially symmetric and there are usually six petals which often have a rumpled appearance.

The family is also remarkable for heterostyly which will be discussed below.

Habit

Purple Loosestrife or Lythrum salicaria forms tall spires of purple flowers that are usually found close to water. The stems are solid, stiff and hairy and can grow up to 6 feet high. At the top, there can be multiple inflorescences. 

Leaves

The leaves are arranged opposite or sometimes in whorls of three. They are sessile or only have a short and indistinct petiole. The blade is 4-7cms long, lanceolate in shape With short hairs on both sides.

Inflorescence and flowers

The purple flowers form tightly packed circles around the stem, a so-called spike. Initially, the flowers are tightly arranged but as the stem grows the intervals between rings of flowers gets greater and the inflorescence can take on a slightly straggly appearance.

Individual flowers are radially symmetric and about 15mm across. The calyx is cup-shaped, reddish in colour and covered with contrasting white hairs.  It has many ribs ending in somewhat irregular teeth. There are six purplish petals that fuse at the base to form a corolla tube. The free part of the petal has a loose wavy appearance.

There are 10 anthers if want to count, an inferior ovary and a style that comes in three different lengths.

So far very beautiful but what was there that was sufficient to drive Charles Darwin to distraction?
Well, you have to look more closely at the sex organs. You may remember from the primrose blog (March 2020) that there are two morphs one with a long style and short stamens and the other with a short style and long stamens; so-called pin and thrum flowers. Well, in the case of purple loosestrife it is a bit more complicated than that. If you look carefully you can see three different forms of the flower.
1. A short-styled morph in which the style and stigma are contained within the corolla tube and is accompanied by medium length stamens with yellow anthers and long stamens with pinks stamens and greenish anthers - see photograph below.

Short styled morph. The stigma is just visible within the corolla tube. The medium-length stamens 
with yellow anthers and long stamens with pink filaments and green anthers can be seen.

2. A medium-styled morph in which two sets of stamens five with long pink filaments and green anthers and five with short filaments and yellow anthers - see photograph x below.

Medium styled morph. Greenish stigma accompanied by 5 short stamens with yellow anthers
and 5 long stamens with green anthers

3. A long-styled morph. The style protrudes well beyond the corolla cup and as you might expect short stamens and medium stamens...

Long-styled morph. The style protrudes well beyond the corolla cup.
Short and medium-length stamens with yellow anthers are visible.

To sum up, there are three different lengths of style and each flower has two sets of 5 stamens that are of different lengths to the style. So the short-styled morph will have 5 medium and 5 long stamens, the medium styled morph has 5 short and 5 long stamens and finally, the long-styled morph has 5 medium and 5 long stamens. This means you never see a style with stamens of the same length. In addition, if you look closely you will see that the stamens are of a different colour - the long stamens have greenish anthers the medium and short stamens have yellow anthers
As you may have guessed, this is a mechanism to reduce the risk of self-fertilisation and encourage outcrossing. Darwin realised that there are18 possible crosses but he showed by experiment that there are six 'legitimate' crosses that produce perfectly fertile seeds.
1) When a long style flower is pollinated by a long stamen from either the short or medium style morphs. (2)
2) When the medium style flower is pollinated from medium length stamens from either the short or long-styled morphs (2)
3) When a short style flower is pollinated by short stamens from either the long-styled or medium-styled morph (2)
In short, each style must be fertilised by stamens of the same length as itself for complete fertility and of necessity, these come from a different plant.

What might the advantage of this approach be over the simpler two morphs we saw in the case of primroses? It may increase the probability of successful outcrossing because two out of three morphs could pollinate in an optimal fashion. 

Where to find purple loosestrife

In Leazes Park you can find it in the floating beds at the western edge of the lake, growing through the block paving surrounding the lake (at least until it is strimmed almost to oblivion), in the marshy area just north of the tennis courts and in the marshy wild area at the north end of Leazes Park. There is lots of it about at the right time of year and there is no reason not to take a good look at it.

You can enjoy Purple Loosestrife on many levels. You can simply go out there and enjoy the elegant purple spires of colour in the late summer. Alternatively, you could look closely at the corolla tubes of individual flowers, put yourself in Charles Darwin's shoes and ponder on the reason why The different forms of the flower?