Recall that in the Ames Test, potential mutagens are tested for their ability to revert His− auxotrophs to His+. Any particular his- mutation present in a His− auxotrophic bacterial strain would be able to be reverted only by some mutagens and not others. Yet, the purpose of the Ames Test is to determine whether or not a specific substance is a mutagen of any type. How is the Ames Test actually performed to circumvent this problem?

To work around this problem, the Ames test is done as follows:

A petri dish is seeded with salmonella colonies, moreover, in that petri dish a little chemical is added in the center. A culture medium is also added to the plate where agar is one of the ingredients and biotin and histidine have been mixed.

Only bacteria that show some kind of mutation will grow on these agar plates. This is because histidine has the function of allowing cell divisions necessary only for cases in which mutagenesis is necessary to occur.

acid rain is the answer

clearcutting reproduces a new even-aged stand by completely removing the mature stand.

clearcutting as a silvicultural treatment has several characteristics:

it causes a sudden environmental change

it removes the seed source from the regeneration area

the configuration of the clearcut may affect seed dispersal

it begins a new rotation

it will eliminate some pests that require forest cover

overall, it temporarily removes the forest cover, transforming the forest community and environment.

clearcutting followed by artificial (seeded or planted) regeneration has several advantages:

we can avoid delay in restocking the site

we can introduce a selected species, seed source, and genotype

we can control arrangement and spacing

we can achieve uniformity in the new stand

we can overcome the problems associated with securing sufficient natural regeneration.

the shelterwood method involves the removal of most of the mature stand at the end of the rotation, but a portion of the mature stand is left standing.   the shelterwood method serves three basic purposes:

to prepare the stand for production of abundant seed

to modify the environment in a way that promotes germination and survival of the selected species

to build up the amount and size of advance regeneration to ensure the prompt restocking of the new stand following overstory removal.

the shelterwood method involves a sequence of 3 cuttings:

preparatory cuttings: make the seed trees more vigorous and set the stage for regeneration. see our thinning page for more information.

establishment/seed cuttings: open up enough vacant growing space to allow establishment of the new regeneration.

removal cuttings: uncover the new crop to allow it to fill the growing space.

the residual trees in the shelterwood must:

be sturdy and windfirm

be able to survive exposure

flower and reproduce seed

be the best trees of the mature stand

keep in mind that the shelter trees must not occupy the entire site.   a void must be created in order to make room for the new regeneration. the amount of shelter to leave will depend on the following factors:

species characteristics (seeding and shelter requirements)

the number of cuttings in the cutting sequence (1, 2, or 3)

landowner objectives

the size of the area to be regenerated

the final removal harvest level required to be profitable

the shelter must be removed before it impedes the growth of the new stand or threatens its survival.