QUESTION: 
What will they do if there is evidence of plant life? 

ANSWER from Jack Farmer on February 25, 1997:
If we do find evidence of past life on Mars, we expect it will be simple
bacterial forms and not the more complex forms represented by plants
(remember that most plants are multicelled forms that are fairly large and
complex compared to bacteria).  On Earth we do have simple unicelled plants
called diatoms which, in principle, could be found as fossils on Mars.  But
is often confusion about what is meant by the word 'plant'. Some people
have used the term to describe any organism that can photosynthesize.  But
many bacteria, all of the cyanobacteria, and the protistans (eukarya) can
photosynthsize, and they are clearly all potential models for Martian life
because they are comparatively small and simple forms.

To summarize, I guess the answer to your question must begin with what you
actually mean by 'plant life'.  If you mean plant life in the strict
biological sense of the word, then I could say it is not likely we will
find large and complex  forms like trees, grasses, etc.  But if you mean
for plant life to encompass only simple unicelled forms that can
photosynthesize, then we have a reasonable chance of finding something like
that on Mars.

Now remember that when plants photosynthesize, they combine water and
carbon dioxide to form carbonhydrates and oxygen.  So if we did find
evidence for simple plant fossils on Mars, we would be extremely excited
and  ask the next question: Given such life forms arose on that planet,
what became of them and all the oxygen they produced?