Leaves found in moderate or mesic climates
The 'typical" leaf has 2-4 layers of palisade mesophyll
cells which are packed with chloroplasts
to trap the suns energy for photosynthesis. Generally there are no more than 1-2 layers of
epidermis on either surface, with stomata found on the lower layer.
The small veins are covered with a single layer of parenchyma cells. The spongy mesophyll
makes up half to 2/3's of the leaf volume, its job to permit gas exchange and to aid cooling.
The slide of the
privet bush leaf we see below indicates it has a high
probabiity of growing in a mesic area. What characteristics indicate this?
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Below are 2 interesting abstracts from recent papers in Nature
concerning the evolution of
leaves and the potential reason why specific primate lines have evolved color vision
in response to leaf color....
evolution: Hungry primates see red
In the tropics, the red ones are more juicy.
Manchester United fans can follow their players' red jerseys on a green football pitch probably thanks to hungry primates. The three-colour 'trichromatic' vision that allows us to tell red from green, may have helped our primate ancestors select the most tender, tasty leaves in the jungle, research from the University of Hong Kong suggests.
Studying primates' food preferences in the Kibale National Park in Uganda, Nathaniel Dominy and Peter Lucas found that three-colour vision is only crucial for choosing tender leaves1. For the past 100 years or so, the prevailing wisdom has been that trichromatic vision endured because it allowed primates to spot fruit high in a green canopy.
"It's very exciting work," says Hannah Buchanan-Smith, who works on primate vision at the University of Stirling in Scotland. "It overturns what we've assumed for a long time."
All Old World monkeys, apes and humans (Catarrhine primates), and one New World group the howler monkey routinely have trichromatic vision. These primates can distinguish blue/yellow, red and green light. Trichromatic vision also occurs to varying degrees in other New World monkeys. All other mammals are dichromatic, they cannot tell red from green light.
Dominy and Lucas documented the eating habits of chimpanzees (Pan troglodytes), black and white colobus (Colobus guereza), red colobus (Procolobus badius) and red-tailed monkeys (Cercopithecus ascanius). They then analysed the colour, chemical content and toughness of the primates' chosen foods.
The type of light reflected by the vast majority of fruit sampled could have been distinguished from background leaves by dichromatic animals. The only food that could not be distinguished from ordinary green without trichromatic vision were tender, often red, new leaves of tropical plants. "You can only detect [these] with the redgreen channel," Dominy explains.
The young leaves of about half of Africa's plants are red; in South and Central America, where trichromacy is less prevalent, the figure is nearer a third. Younger, red leaves are more tender, digestible and protein-rich.
Few primates eat solely leaves, but all of them do so when other food is scarce. So eating the most nutritious and easily digestible leaves would be a considerable advantage, Dominy and Lucas argue. This, they say, implies that leaf colour probably drove the evolution of trichromatic vision, not the ability to spot fruits among leaves high in the forest canopy.
This work with Old World primates, which are all trichromatic, is the first to identify an alternative mechanism for the development of three-colour vision. But more research must be done before broad conclusions can be drawn about the evolutionary underpinnings of trichromacy in all primates, Buchanan-Smith cautions.
By observing the feeding habits of New World primates, the majority of which are dichromatic, more might be learnt about why some primates see red. "A direct comparison between dichromats and trichromats is something that needs to be done," says Buchanan-Smith.
1. Dominy, N. J. & Lucas, P. W. The ecological importance of trichromatic colour vision in primates. Nature 410, 363366 (2001).
relics: Fall put leaves on trees
A carbon dioxide slump put leaves on the trees
For the first 40 million years of their existence, land plants didn't bother to make leaves just green stems and small, spiny protrusions. Leaves only evolved, researchers now suggest, when a drop in the amount of carbon dioxide in the atmosphere meant that the benefits of intercepting more light began to outweigh the dangers of overheating.
From living and fossil plants, and geochemical information about past environments, David Beerling, of the University of Sheffield, UK, and colleagues have reconstructed the period when leafiness became a viable lifestyle1.
"We don't really know why leaves evolved," says Paul Kenrick, a plant palaeontologist at London's Natural History Museum, "nobody's thought of carbon dioxide as being a factor in leaf evolution before."
The amount of carbon dioxide in the atmosphere is reflected in the number of 'stomata' that plants produce. Stomata are the pores that let gases in and out, and out of which water evaporates, cooling the plant.
The earliest plants had very few stomata. This served them well in a carbon dioxide-rich atmosphere, and helped to stop them drying out. Any leaves produced by such non-porous plant life would have quickly perished from overheating, Beerling's team calculate.
Leafless plants, though, were preparing the world for their successors: about 380 million years ago, plants seems to have brought about a 90% drop in the atmospheric concentration of carbon dioxide. Plants lock up carbon in their woody tissues and roots, and by breaking up rocks, increasing the rate at which carbon passes out of the atmosphere.
The new atmosphere changed the economics of photosynthesis. Plants developed lots of stomata making them better at taking up carbon dioxide, and at staying cool. Branched green stems became webbed, and highly divided, blade-like leaves developed, from which one-piece leaves quickly followed.
Hand-in-hand with leaf evolution came sophisticated systems to transport water through the plant. This better plumbing may also have allowed plants to increase in size and complexity.
Although the new leafy plants had to work harder to get carbon dioxide, they could capture more solar energy without overheating. So plant productivity increased, and animal life would have had more food.
Of course, it's hard to know what the world was like 400 million years ago, harder still to work out how the life of the time adapted to it. "We are assuming that modern and ancient plants had the same responses," Beerling admits. Stomata and plants' responses to carbon dioxide seem not to have changed much in the past 400 million years: today, plants are making fewer stomata in response to the increased carbon dioxide in the atmosphere.
Kenrick comments that, with more and better plant fossils from between 410 and 370 million years ago, researchers should be able to back up the relationship between leaves and carbon dioxide, or to shoot it down. One would expect, for example, to see leaves first appear in plants from cooler, temperate regions, where overheating is less of a danger. The ball, says Kenrick, is now in the fossil-hunters' court.
1. Beerling, D. J., Osborne, C. P. & Chaloner, W. G. Evolution of leaf-form in land plants linked to atmospheric CO2 decline in the late Palaeozoic era. Nature 410, 352354 (2001).
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