Cool Black, Warm White

Rose Genetics Flower Pigments
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1

A famous rose breeder (I forget which one) reportedly stated that if he ever succeeded in raising a black rose, it would burn up in the field.

Maybe not.

About 20 years in southern California I raised some pansies that were literally black as coal. The only hint of color was a tiny violet halo around a tinier yellow eye. In the past couple of years here in Kentucky I have seen petunias that are not pansy-black, but still very dark. Like the pansies, they are not bothered by direct sunlight. Then there “black” irises, daylilies, roses, and other flowers that are about as indifferent to bright light.

Several years ago I visited the Heritage Garden in San Jose on a particularly hot and sunny day. I made a point of comparing roses of different color. Some of the darkest varieties (e.g., Lavaglut) looked as fresh as a Spring morning, while several pale varieties were limp.

One quality that seems to be shared by the more durable black and near-black flowers is a velvety sheen. This sheen is the result of a papillate epidermis. (See Kay, 1981 for illustrations.) Each cell is raised above the surface in the form of a rounded cone. This extension gives the cells much more surface area from which to radiate heat, without increasing the quantity of light that reaches them. In other words, one square centimeter of a papillate epidermis receives the same quantity of light as the same area of a petal with a flat epidermis. Thus, the papilla act like the “wings” on a radiator, dissipating heat. Cells with a flat surface radiate less heat while transmitting more to the underlying tissue.

I read about “Cool Black” and “Warm White” many years ago, but only recently found the paper that I must have read. It may have been reprinted, or heavily quoted, in some magazine.

Hay: Solar Radiation and Color Adaptation. International Congress on the Sun in the Service of Mankind. (1973)

The paper is well worth reading. The author points out that the white fur of arctic animals may not be purely a matter of camouflage. The white hairs act as light pipes, carrying light down to the skin. Dark coloration of fur, on the other hand, tends to keep an animal cooler when it is exposed to tropical sunlight.

He also discussed iridescence. When we see the brilliant plumage of a tropical bird we may suppose that the selectively reflected light is the important point. Maybe the real reason for iridescence (in some cases) is the anti-microbial properties of the light that is selectively retained and carried to the skin.

In regards to roses and other flowers, breeders breeding for the darkest colors probably should also aim for “velvet”, and not only because it looks so fine.

Karl

2

It’s interesting to note that examples can often be found in nature to support multiple points of view :wink:

Every student of genetics is taught about the environmentally influenced expression of fur colour in Himalayan rabbits. They are black on their extremities (ears, nose, feet and tail). The reason given for this is that these regions lose heat more easily than the core does and the black colour of the fur in colder climates acts like a heat sink. This allows them to collect heat which consequently helps to heat the blood returning to the body to maintain their core temperature. If you shave the white fur off their back and strap an ice pack to the shaved area the hair will grow back black in response to the lower temperatures. With the tropical birds example I would ask the question why a lot of the iridescence is often located on plummage extensions that cannot physically relay light to the skin? I also note that the fur in artic animals such as polar bears is hollow to facilitate the optic fibre effect that may not occur to the same extent if the hair shaft was white and solid.

I also wonder whether there might be multiple causes of the black velvet look in roses or there might be some spectral influence caused by the pressence of different pigments because here the miniature ‘Black Jade’ is about as black as they get (see my photo on HMF: 'Black Jade ' Rose Photo ), and they quickly burn to a crisp our sun.

3

Simon,

No doubt there are other factors. As I recall, the ‘Black Jade’ I saw at the Heritage Rose Garden in San Jose, CA was shaded by its taller neighbors, so I didn’t get a chance to see whether it burned. ‘Lavaglut’, on the other hand, always attracted my attention, so I was more likely to observe it in any season.

4

“Dark coloration of fur, on the other hand, tends to keep an animal cooler when it is exposed to tropical sunlight.”

I will tell that to my black horse next time she has to stand out in the pasture on a 100 degree day!

B.

[attachment 945 SpiritMay2007.jpg]

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I’m also curious to know why females in tropical bird species often lack iridescence completely, or posses much in the way of melanin in their plumage at all? If anyone needs it more for the control of pathogens it would be the females who spend much time sitting in a broody torpor on a nest in an environment that encourages these types of pathogens. When I incubate chicken eggs I need to keep the humidity at 50% for 18 of the 21 days and then increase the humidity to 80% over the last three days. This correlates with the broody hen’s own humidity control against the eggs which helps to prevent the chicks from sticking to the insides of the shells during hatching and assists them to ‘dry’ more cleanly too. They wouldn’t be wanting them to hatch into a pathogen loaded environment before their immune systems are strong enough to cope with them. Too many questions that raise too many counter questions I think… It’s interesting though… I feel an experiment coming up with the kids at school to test this idea!

6

A lot of this is out of my league, but from memory, black the color absorbs heat and white reflects it.

On a side note, love the horse, great looking animal.

7

Very interesting thread, thank you! My impression is the iridescence of the bird plumage may have multiple benefits. It not only possibly attracts female mates, but is more visible to predators, enabling the male to lead them away from the nest and female. Being located at the extremities, it should also attract the aim of the predator toward the ends of long feathers, giving the bird better chances of avoiding capture.

I don’t know about the dark fur keeping the animals cooler in sun, but in humans, dark hair generally doesn’t have to be as fine and dense as light colored hair does to provide the sun protection (as opposed to HEAT protection). Lighter colored hair doesn’t block the radiation as efficiently as dark pigments do, so much more of it is usually required to provide that solar protection. Light colored eyes are usually more adversely affected by radiation than darker ones, as is lighter pigmented skin.

There are very few “black” blooms here which don’t fry quickly. Dark reds, even those not terribly dark like Lincoln, Velvet Fragrance, Legend will fry quickly. Most violet tones go off very quickly, too, no matter how small or upright their petals are. Black Baccara fries in a few minutes when it opens on a mid nineties and higher day, particularly when it is arid and with any breezes to wind. “Petal substance” has a lot to do with how quickly a petal fries in this kind of heat. Lyn Griffith, whom you all probably know from HMF, has a real heat sink in her back yard. She’s been studying what seems to contribute to making roses with flowers that last any length of time in her hot rear garden. How dense the foliage mass of the plant is was one rather interesting potential factor. As if the large foliage mass not only kept the plant cooler, but drew enough water from the ground to keep it pumped into the flower, helping to prevent it from crisping as quickly.

FWIW, both Lavaglut, Black Jade and Black Jack fry here when it is hot and dry.

8

David,

Of course! The point is that when the surface is discontinuous (fibrous, porous, particulate) much of the heat is re-radiated rather than transmitted to whatever lies beneath. My first experience with this, years before I read the article, came while I was playing baseball as a kid. I brushed back my hair and felt that it was hot. My scalp, however, was protected. Now that I’m older, I experience more of the “warm white” effect.

Bandit, my late, red-nosed pit bull, was very sensitive to bright sunlight on a hot day. See how he squints. When we took walks, we had to stop in the shade frequently so he wouldn’t overheat.

[attachment 946 Bandit.jpg]

A rose, or any flower, with a papillate epidermis has more surface area than a flower with a flat epidermis. It can re-radiate more heat. If the two flowers are similarly colored and of comparable ancestry, the papillate petals should be relatively cooler than the smooth ones. Just as the black sheep that Hay mentioned enjoyed a relatively lower body temperature than the white ones given the same exposure.

Last year I visited the rose garden in the Lexington Arboretum to take notes on which varieties were blooming despite the heat. I was pleased to see that a couple of my old friends, ‘Diamond Jubilee’ and ‘Rubaiyat’, were flowering freely. Neither benefits from the “cool black” effect, yet both were holding up nicely. Perhaps the fact that both varieties, and a few others, were growing and flowering in the heat gave them an additional edge over others (of whatever color) that prefer a somewhat lower temperature for optimum growth. ‘Diamond Jubilee’ inherits its love for heat from ‘Marechal Niel’. And ‘Rubaiyat’ first came to my attention because Cooper (1970) mentioned it as a variety that never produced blind wood in his garden.

Kim: Do any light colored roses in your collection burn up in hot, dry weather?

Karl

9

Hi Karl, yes, many lighter colored blooms crisp fairly easily. Too Cute, with its very soft, fragrant petals, fries quickly. Mrs. Charles Bell fries around the edges. White Delight doesn’t crisp as quickly nor as severely, and also doesn’t have the scent factor. Cl Yellow Sweetheart turns crisp quite quickly. Leonie Lamesch opens crisp on hot, arid days. Bridal White crisps easily, though Snowbird doesn’t. Bridal White is without scent, but Snowbird has some and they are very close to one another in location (maybe two or three feet away from one another as everything has to be crammed in to fit the level space). The Renae seedlings (sisters of Annie Laurie McDowell) and Opal Brunner fry quickly as they also have very fragrant, very soft petals. Loyal Friend doesn’t fry. Apricot Bells fries immediately, Patricia Beucher doesn’t. Queen Elizabeth usually only fries on intensely hot, sunny, dry days and mostly where she receives focused radiated and reflected heat from the white house wall, though Gardens of the World, about four feet away in the exact situation, seldom fries. Edith Schurr fries quickly.

Reve d’Or fries immediately. Lamarque takes a bit longer and is actually in a more severe situation. Porcelain Rose seldom fries and is in a terrible situation for heat and continuous sun. It does feel as if there is quite a strong correlation between petal substance and relative fragrance; hardness of pith; density and durability of foliage and petal color with how quickly and easily the petals crisp.

10

Linda Campell is one of the fuzziest roses I have ever grown, and it does pretty well for a red.

My seedling of Raven x Preference is Lavaglut red, but it just gets polka dots when it is hot. Raven doesnt seem to fry as bad as its parent, Lavaglut, which I am guessing is owed to the orange minis it was bred from.

Ketchup and Mustard is going to be translocated from my sight by autumn. It was beautiful until it got warm, which is when it decided its leaves and petals were worse than a mauve rose’s leaves/petals. It is now leafless, but not from disease, and the blooms became miniature and pink, lol. Neither of its parents did this here =/ … LEAFLESS! Seriously, lol. Ive never seen that before.

11

Gee, you must have got a lemon, mine has seven flowers. I have to agree, the flower size is now like a mini and no hips so far.

Neil