More colour questions

[Ipso Facto]

Following the great success in getting my questions answered about horse chestnuts more colourful questions have come to mind - particularly concerning the hawthorn. There are red ones, white ones and pink ones. Are they all related? Why are they different?

[Hornbeam]

The two main Hawthorns are the Common Hawthorn (Cratageus monogyma) and the Midland Hawthorn (Cratageus laevigata) which is a much smaller shrub rather than a tree. The leaves of the first are deeply lobed and those of the second have shallow lobes. The two types hybridise freely where they grow in the same area. Flowers are mainly white but can be light pink - especially on the Midland Hawthorn. There are lots of cultivated hawthorns such as Pink May (Common), Paul's Scarlet (Midland) and the double "Punicea Flore Plena". Then there are foreign hawthorns which have been introduced. There are over 200 species of Hawthorn!

[Ipso Facto]

Originally Posted by Hornbeam

The two main Hawthorns are the Common Hawthorn (Cratageus monogyma) and the Midland Hawthorn (Cratageus laevigata) which is a much smaller shrub rather than a tree. The leaves of the first are deeply lobed and those of the second have shallow lobes. The two types hybridise freely where they grow in the same area. Flowers are mainly white but can be light pink - especially on the Midland Hawthorn. There are lots of cultivated hawthorns such as Pink May (Common), Paul's Scarlet (Midland) and the double "Punicea Flore Plena". Then there are foreign hawthorns which have been introduced. There are over 200 species of Hawthorn!

Thanks for that Hornbeam, some great facts, but I am still left with a multitude of questions that I hope someone can answer.

Is the only way to differentiate between the species by some function of the degree of "lobedness" and by analysing the hue using a colorimiter? What is it that really makes the species distinct?

Where did the pink/red flowers come from? Have they always been present in the UK or was the redness introduced from elsewhere?

Why is it called the "Midland" hawthorn?

Are the 200 species totally separate different or do they include the hybrids?

Why am I suddenly so excited by all of this???

[Paul mabbott]

The styles: usually 1 on C. monogyna (thus its name!) but 1-3, usually 2 on C. laevigata - this sounds very iffy but looking at several flowers (on the same tree), you will see that most have either 1 or 2 styles. Not much use after flowering, admittedly ....

[thunder]

Quote
Is the only way to differentiate between the species by some function of the degree of "lobedness" and by analysing the hue using a colorimiter?
Endquote

As Paul has said, counting the styles in the flowers will help, and looking at a lot of leaves to see how lobed they are. Colour is not necessarily very helpful, though deeply coloured ones are likely to be some sort of hybrid or cultivated variety. It's unclear to me where colour in hawthorns came from - but probably either from a natural genetic modification or from hybridisation with a foreign species.

Quote
What is it that really makes the species distinct?
Endquote

In general, species do not breed with other species, or if they do they don't produce fertile offspring. But in the case of Common and Midland Hawthorn this definition breaks down, as they do produce fertile offspring! The idea of a "species" is purely a human one, of course.

Quote
Where did the pink/red flowers come from? Have they always been present in the UK or was the redness introduced from elsewhere?
Endquote

See above.

Quote
Why is it called the "Midland" hawthorn?
Endquote

Because it is commonest in the Midlands, though it has been planted a lot and the true native area is uncertain.

henrya

[Ipso Facto]

Thanks henrya.

It's such a beautiful evening that I have just been out comparing degrees of "lobedness" - the plot thickens!



The above scanned image is divided into 4 pairs: each pair is from the same tree and all 4 trees were in the same vicinity - in all cases I have tried to take a representative (though small) sample, i.e. I haven't just chosen mal-formed leaves. By comparing blossoms I would have said they were of the same species but the shape of the lobes within a pair can appear different (age??) and sample 3 looks decidedly different - can it really be the same species?

How do you experts do this?

[thunder]

It's a cop-out, I know, but I think you have hybrids here. Did you count the numver of styles?

henrya

[Paul mabbott]

Originally Posted by Paul mabbott

The styles: usually 1 on C. monogyna (thus its name!) but 1-3, usually 2 on C. laevigata - this sounds very iffy but looking at several flowers (on the same tree), you will see that most have either 1 or 2 styles. Not much use after flowering, admittedly ....

Answering my own question - when you get the fruits, I assume that 'Midland hawthorn' has more than one seed/pit?
(This is comparable to apples v cherries - the latter have one style and thus one pit while the former have multiple styles and several pips).

[Paul mabbott]

Tricky points!
I think the colour is a bit of a red (or pink) herring - this is just genetic variability which has nothing to do with species - many other white flowering plants have the occasional pink flowering individual ... and vice versa (Herb Robert quite often throw up plants with white flowers, for instance). Flower colour is one of the least reliable factors for many species.

Yes, the species concept is convenient but it is also real in most cases (species are, as with your definition, populations that cannot interbreed and produce viable offspring). This is easier to define with animals than with plants because, in plants, of the process of polyploidy and other strangeness - viable seeds can be formed without sexual reproduction so it's not always easy to test this!

And I don't want to get involved in that! Returning to Crataegus leaves, I'll quote CT&W:
.. but I won't use all the abbreviations!
- laevigata leaves of short-shoots lobed less than half-way to the midrib, the lobes broader than long and usually rounded....
- monogyna leaves of short-shoots more than half-way to the midrib, the lobes longer than broad +/1 triangular ......

So that's straightforward ( ) - have you got "short-shoots" seems to be the first question?

Originally Posted by thunder

Quote
Is the only way to differentiate between the species by some function of the degree of "lobedness" and by analysing the hue using a colorimiter?
Endquote

As Paul has said, counting the styles in the flowers will help, and looking at a lot of leaves to see how lobed they are. Colour is not necessarily very helpful, though deeply coloured ones are likely to be some sort of hybrid or cultivated variety. It's unclear to me where colour in hawthorns came from - but probably either from a natural genetic modification or from hybridisation with a foreign species.

Quote
What is it that really makes the species distinct?
Endquote

In general, species do not breed with other species, or if they do they don't produce fertile offspring. But in the case of Common and Midland Hawthorn this definition breaks down, as they do produce fertile offspring! The idea of a "species" is purely a human one, of course.

Quote
Where did the pink/red flowers come from? Have they always been present in the UK or was the redness introduced from elsewhere?
Endquote

See above.

Quote
Why is it called the "Midland" hawthorn?
Endquote

Because it is commonest in the Midlands, though it has been planted a lot and the true native area is uncertain.

henrya

[Hornbeam]

The hybrids are confusing, There is no confusion possible when you see the leaves of the true common hawthorn and the true midland side by side. The midland hawthorn is so much smaller than the common and more like a shrub.

Flowers and leaves of the common hawthorn



Flowers and leaves of the midland hawthorn - note the shallow lobes and compare with those on the common hawthorn



As regards colours - why are some roses red and others white? The hawthorn is a member of the rose family also. The deep red hawthorns are obtained by selective breeding.