#5 - Only some strains of the tobacco plant are naturally

[andriana.caban]

Hi!

I know why (B) is correct but there are some things that still confuse me for (E).

I was under the impression that each strand of the tobacco plant naturally produce SA based on the authors second sentence, "when resistant strains...of naturally occurring SA...no such increase occurred in non resistant plants". When injected with the virus, resistant plants saw an exponential increase in SA while the non resistant plants saw no increase.

Would this not show that (E) is correct?

(E) says the production of SA in certain strands of tobacco plants can be increased (the experiment says we injected SA in non resistant strands) and the strains are made resistant to the TM virus (for the plants that we injected SA in, we did not see signs of infection). Therefore, SA was increased and the plants showed no signs on symptoms.

The only reason why I understand (B) to be correct over (E) is due to the fact the author stated the plants "showed no signs of infection" we cannot assume that they are necessarily diseased or not. Maybe they're not showing signs now but will later?


Please explain why (E) is wrong or if my understanding of the stimulus is incorrect?

[Adam Tyson]

The issue with answer E, andriana.caban, is that we don't know if the plants that produce SA can be made to produce more. We can inject some into them, and that looks like it helps them, but that doesn't mean that we can encourage the plants to start churning out more on their own. It's not whether the amount can be increased - we know it can, by injecting more into them - but whether the production (the amount they make on their own) can be increased. Maybe it can, through genetic engineering or some kind of biological therapy, but the stimulus gives us no information about that, and so we cannot conclude that it is possible.

[anureet]

Hello,
I just to clarify option B. It is not causal reasoning it just says it plays a part in plants naturally resisting the disease which is different from saying salicylic acid causes plants to naturally resist the disease caused by the tobacco mosaic virus.

Regards
Anureet Bhatti

[Adam Tyson]

That's what we might call soft causal reasoning, Anureet. The conclusion we can draw is that salicylic acid plays at least some role in protecting the plants that produce it, and that means that the acid is at least partially causing their resistance to illness. There's definitely some causal reasoning involved, but it's not the kind of extreme causal reasoning that we see in other arguments where the author unreasonably concludes that the observed correlation means an exclusive or absolute causal relationship. Partial causes - contributing factors - are a lot easier to accept and to infer, and in this case it's the best inference we can draw from these facts.

[gabe_katz)=_]

Hi,

I put answer D thinking that it would show that salicylic acid is present when a tobacco plant is resistant to tobacco mosaic virus, but not necessarily the cause of the tobacco plant's resistance (I.e. I thought we could assume correlation but not necessarily causation). It turns out that answer B is correct rather than answer D. Can you explain to me why that is the case?

Thanks,
Gabe

[Luke Haqq]

Hi Gabe!

You are right to keep in mind that correlation does not necessarily imply causation. Causation is frequently tested in logical reasoning, so it's worthwhile to keep on guard for errors in causal reasoning.

In this question, the stimulus provides enough information to understand at least a piece of the causation in play. In particular, it states, "When resistant strains were experimentally infected with the virus, levels of naturally occurring salicylic acid in these plants increased fivefold." This language conclusively tells the reader that the increase in salicylic acid occurs after the infection.

That difference explains why (B) is correct rather than (D). Answer choice (D) states, "It is possible to test an uninfected tobacco plant for resistance to tobacco mosaic virus by measuring the level of salicylic acid it contains." This is indicating that salicylic acid is present before the infection, which there isn't support for in the stimulus. By contrast, answer choice (B) states, "Producing salicylic acid is at least part of the mechanism by which some tobacco plants naturally resist the disease caused by tobacco mosaic virus." This accords with the timing indicated in the stimulus, namely, that infection is followed by production of salicylic acid.