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版主你好～ 螞蟻和樹 是這篇嗎？ 感謝～～
乳草蚂蚁蚜虫
Who’s the boss? Milkweed is the boss.
Milkweed plants engage in a helpful bit of mutualism with the aphids and ants who take up residence on them. Aphids feed on the milkweed’s sap, then secrete honeydew, which ants eat. The ants, in turn, are the muscle of the operation—they help both the plants and the aphids by fighting off potential predators like caterpillars. The partnership goes three ways, but the power is not equal—milkweed is in control.
Researchers Kailen Mooney and Anurag Agrawal recently found that the milkweed can manipulate the relationship between ants and aphids, altering the dynamics for its own good. The scientists planted 32 groups of milkweed, with each group containing 10 siblings from the same family, in a field full of ants. In 20 of the 32 milkweed groups, the presence of ants was a big boon for aphids—the aphid population increased by 150 percent compared to plants with no ants to protect them. But in the other 12 groups, the numbers of aphids actually decreased by more than half when ants were around.
So why should the three-way arrangement benefit all parties in most cases, but not in others? Since the researchers divided milkweed into groups according to genetic families, they say the plants’ genetics must be controlling the relationship. Mooney and Agrawal aren’t exactly sure how, but they speculate that the composition of milkweed sap can vary from family to family. Perhaps some families of milkweed just didn’t need as much protection, and therefore allowed fewer aphids to live on them by producing a less inviting sap. After all, this arrangement is not free for the plants—they give up lots of sugar and water for the aphids to live on the plant and induce the ants to come.
However it happens, the scientists say, finding that plants can manipulate the ant-aphid relationship helps to explain why the numbers of aphids and ants can vary so much between one plant and another, a problem that had puzzled biologists before. Now they know—milkweed is making a power grab.
眼球结构与对动静物体敏感的差异
And yet only recently have researchers come to appreciate the profound importance of such “fixational” eye movements. For five decades, a debate has raged about whether the largest of these involuntary movements, the so-called microsaccades, serve any purpose at all. Some scientists have opined that microsaccades might even impair eyesight by blurring it. But recent work has made the strongest case yet that the seminuscule ocular meanderings separate vision from blindness when a person looks out at a stationary world.
Indeed, animal nervous systems have evolved to detect changes in the environment, because spotting differences promotes survival. Motion in the visual field may indicate that a predator is approaching or that prey is escaping. Such changes prompt visual neurons to respond with electrochemical impulses. Unchanging objects do not generally pose a threat, so animal brains – and visual systems – did not evolve to notice them. Frogs are an extreme case. A fly sitting still on the wall is invisible to a frog, as are all static objects. But once the fly is aloft, the frog will immediately detect it and capture it with its tongue.
Frogs cannot see unmoving objects because, as Helmholtz hypothesized, an unchanging stimulus leads to neural adaptation, in which visual neurons adjust their output such that they gradually stop responding. Neural adaptation saves energy but also limits sensory perception. Human visual system does much better than a frog’s at detecting unmoving objects, because human eyes create their own motion. Fixational eye movements shift the entire visual scene across the retina, prodding visual neurons into action and counteracting neural adaptation. They thus prevent stationary objects from fading away.
The results of these experiments, published in 2000 and 2002, showed that microsaccades increased the rate of neural impulses generated by both LGN and visual cortex neurons by ushering stationary stimuli, such as the bar of light, in and out of a neuron’s receptive field, the region of visual space that activates it. This finding bolstered the case that microsaccades have an important role in preventing visual fading and maintaining a visible image. And assuming such a role for microsaccades, our neuronal studies of microsaccades also began to crack the visual system’s code for visibility. In our monkey studies we found that microsaccades were more closely associated with rapid bursts of spikes than single spikes from brain neurons, suggesting that bursts of spikes are a signal in the brain that something is visible.
In our experiments, we asked volunteers to perform a version of Troxler’s fading task. Our subjects were to fixate on a small spot while pressing or releasing a button to indicate whether they could see a static peripheral target. The target would vanish and then reappear as each subject naturally fixated more – and then less – at specific times during the course of the experiment. During the task, we measured each person’s fixational eye movements with a high-precision video system.
As we had predicted, the subjects’ microsaccades became sparser, smaller and slower just before the target vanished, indicating that a lack of microsaccades– leads to adaptation and fading. Also consistent with our hypothesis, microsaccades became more numerous, larger and faster right before the peripheral target reappeared. These results, published in 2006, demonstrated for the first time that microsaccades engender visibility when subjects try to fix their gaze on an image and that bigger and faster microsaccades work best for this purpose. And because the eyes are fixating – resting between the larger, voluntary saccades – in the vast majority of the time, microsaccades are critical for most visual perception.
独立书店的生存
While the rapid growth of Barnes & Noble, Borders and Amazon has made the business more centralized, each state's bookselling industry is shaped by matters as varied as climate, geography, regional economy, population growth and local culture.
Washington and Oregon, for example, both have thriving bookselling scenes that some attribute partly to the perpetual drizzle of the Pacific Northwest. As agent Stephanie Griffin said of her Oregon city, “Portland has a huge literary community. It rains a lot so there's plenty of time to read.”
For booksellers in Alaska and Hawaii, geography creates a set of difficulties distinct from those in the 48 contiguous states—from having to wait longer to get books to not being able to attract authors to travel the long distances to their stores. Beyond that, their bookselling environments have turned out to be as different as their weather: Alaska, dotted with towns too small to attract a chain store, has half the population (656,000) but a third more (15) independent bookstores than Hawaii, where Borders is the dominant bookseller, with 14 locations.
Though mass merchandisers have become an important force in bookselling throughout the country, their influence is felt in some areas much more strongly than in others. Wal-Mart, headquartered in Bentonville, Ark., is the biggest bookseller not only in its home state, but also in Mississippi, which, with the lowest average household income in the country, has attracted comparatively few national bookstores.
As for the independents' struggle to survive while competing against the chains, it continues, though most of the indies that are still in the game have developed strategies that enable them to avoid going head-to-head with national retailers on price and selection. That means locating in underserved areas as well as focusing on a niche. Take Indiana, where 97 of its 142 bookstores are independents, most operating in rural communities outside the greater Indianapolis area. “They're specialty and small stores in a state that is primarily agricultural and rural,” said Jim Dana, executive director of the Great Lakes Booksellers Association. “This is a state that—other than Indianapolis and the other cities—really depends on independent bookstores.”
For independent stores located in the shadow of a chain or big-box retailer, finding the right niche is key. “I think we're truly getting into the age of specialty stores,” said Ted Wedel, a Maryland resident who is co-owner of the sales representative group Chesapeake & Hudson. Wedel pointed to Maryland stores such as Atomic Books, specializing in pop culture titles, and Breathe, which focuses on New Age topics.
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发表于 2017-10-15 16:23:34
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