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should never differ in temperature by more than about 5oC. So, in answer to our question whether galls significantly affect the temperatures of the leaves they afflict, about the best we can do is to say "sometimes yes, sometimes no."

We next must ask: would the temperature elevations expected for galled leaves have any effect on the leaf's net photosynthesis? To get there, we need some real values for how temperature affects net photosynthesis in a real plant. Fortunately, data like these are abundantly available for agriculturally important plants, so I have extracted from the literature a net photosynthesis curve for corn, Zea mays (Fig. 9.10). This curve shows aT of about 38oC, with net photosynthesis falling to zero at 12oC and 51oC.

To evaluate the likely effect galls might have, we simply estimate leaf temperatures as in the experiment described above, plot them on this curve, and compare where on the net photosynthesis curve the points lie. It does no good to estimate just any old leaf temperature, though: the estimates must be done systematically. A sensible approach might frame the estimates in the context of the leaf's optimum temperature. Let us then start with the simplest case: set conditions so that the temperature of a leaf unburdened with galls equals the optimum temperature (Fig. 9.10). Under these conditions, galled leaves are 2-3oC warmer than ungalled leaves. Because the net photosynthesis curve is somewhat flattened around T$, such small variations of temperature have little effect on net photosynthesis. Consequently, the small increase of temperature experienced by the galled leaves will have little effect. Leaves with needle galls, for example, have rates of net photosynthesis that are at about 99.3 percent of the optimum. Leaves with spherical galls photosynthesize at a net rate of about 98.7 percent of the optimum.

Leaves do not always function at their optimum temperatures, however—leaf temperature may vary through the day, or from day to day. or from place to place on the plant. To be thorough, our comparison must also consider what happens when leaf tempera needle needle

Figure 9.10 Estimated net carbon flux for normal leaves and leaves with simulated gall disease. Environmental conditions are set so that a normal leaf functions optimally; leaves with galls function somewhat less well.

Leaf temperature (oC)

Figure 9.10 Estimated net carbon flux for normal leaves and leaves with simulated gall disease. Environmental conditions are set so that a normal leaf functions optimally; leaves with galls function somewhat less well.

ture differs from the optimum. Using the optimum temperature as a frame of reference, we set conditions so the ungalled leaf is photosynthesizing at 90 percent of its maximum net rate. Two comparisons must be made, one with leaf temperature cooler than the optimum (Fig. 9.11) and one with leaf temperature warmer (Fig. 9.12).

At these temperatures, we find that galls affect photosynthesis more strongly. Although the actual temperature differences between galled and ungalled leaves are small, their effect on net photosynthesis becomes larger as the change from the optimum increases. At cooler leaf temperatures, the small warming of the galled leaves is sufficient to boost net photosynthesis by a few percentage points: leaves with needle galls should function at about 93 percent efficiency while leaves with spherical galls work with about 94 percent efficiency (Fig. 9.11). At temperatures warmer than T$, the effect of galls is larger (Fig.

9.12), both because the temperature elevations are larger in the galled leaves and because the drop-off in net photosynthesis is steeper at temperatures above the optimum. When ungalled leaves photosynthesize at 90 percent of the optimum, galled leaves function at 76 percent efficiency for needle galls and 71 percent efficiency spherical galls.

The Parasite's Dilemma

In popular culture, parasites are bad things, "freeloaders" who suck their victims dry and throw them away. True, some parasites do just this (like the ichneumon wasps described in Chapter 8), but in general this is really not a very sensible strategy. The host, after all, is the conduit for the flow of energy and materials to the parasite. What serves a parasite's interests far better is to keep the host alive and as healthy as possible for as long as possible.1 The parasite's dilemma, then, is this: how to keep the host healthy enough so that you can continue to steal as much from it as you can?

To illustrate the parasite's dilemma, consider a curi-

1. There are circumstances where using up the host and throwing it away might be a sensible strategy, however. The evolution of a parasite's life history is determined simply by what transmits the parasite's genes most readily into future generations, and not by any attention to the interests of the host. Whether the parasite comes to an accommodation with its host or becomes virulent, killing the host, depends upon how readily transmission from host to host can be accomplished. If transmission of the parasite between hosts is difficult, the best strategy for the parasite is accommodation. On the other hand, if transmission between hosts is easy, then the parasite's interests are less readily served by protecting the host, and virulence will likely be the result. This is one of the reasons why public health officials raise concerns about social policies and behaviors that make it easier to transmit disease organisms from person to person. For example, a hypodermic needle exchange program for drug abusers that does not simultaneously prevent needle sharing among abusers may transmit the more virulent strains of disease organisms more readily than a program that forces people to use the needles in the supervised environment of a hospital or clinic. The end result of a poorly supervised needle exchange program may be an increase in the virulence of otherwise manageable disease organisms.

needle galls (92.93%)

needle galls (92.93%)

Figure 9.11 Estimated net carbon flux for normal leaves and leaves with simulated gall disease at cool temperatures (< 38°C). A normal leaf in this environment functions at 90 percent of optimum, and the warmer leaves with galls function at a slightly higher level.

ous political situation that has developed in the United States in the 1990s. During this time, our state and local governments took it upon themselves to impose substantial financial burdens on tobacco companies. The ostensible reason is to force tobacco companies to bear the social costs of their products. Arguably, the real purpose is to punish Big Tobacco for its sins, most notably the resolute indifference of cigarette makers toward those who—whether they be responsible adults or babes in their cribs—consume their products.

Here I am thinking of the government as the parasite and the tobacco industry as the host. I realize the casting is not politically correct, but the analogy doesn't really work the other way. Now government can, if it so chooses, extract so much money from tobacco companies that they go out of business. However righteous this course might be, though, it poses real problems for government: the sums of money being extracted from tobacco companies are already so enormous that many government programs could not be funded without this cash. If the tobacco industry goes belly-up, the loss in tax revenues would be painful, perhaps devastating. So government now finds itself in the rather ludicrous position of having to keep the demon tobacco industry alive and flourishing. This is the parasite's dilemma.

The usual way out of the parasite's dilemma is to live off margin, as bookies, mobsters, stockbrokers, and politicians do: you insert yourself into a stream of money and skim off a little as it passes by, but not too much. To make more money off the stream, you could increase your margin, but that increases the risk the stream of money will dry up. A more sophisticated way is to get people to give you more money voluntarily. State lotteries are a good example of this. Most governments already tax their citizens to the maxi

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