Section B
Directions: There are 2 passages in this section．Each passage is followed by some questions or unfinished statements．For each of them there are four choices marked A, B, C and D．You should decide on the best choice.
If you go down to the woods today, you may meet high-tech trees-genetically modified to speed their growth or improve the quality of their wood．Genetically-engineered food crops have become increasingly common, albeit controversial, over the past ten years．But genetic engineering of trees has lagged behind．
Part of the reason is technical．Understanding, and then altering, the genes of a big pine tree are more complex than creating a better tomato．While tomatoes sprout happily, and rapidly, in the laboratory, growing a whole tree from a single, genetically altered cell in a test tube is a tricky process that takes years, not months．Moreover, little is known about tree genes．Some trees, such as pine trees, have a lot of DNA-roughly ten times as much as human．And, where- as the Human Genome Project is more than halfway through its task of isolating and sequencing the estimated 100,000 genes in human cells, similar efforts to yze tree genes are still just saplings (幼苗)．
Given the large number of tree genes and the little that is known about them, tree engineers are starting with a search for genetic 'markers'．The first step is to isolate DNA from trees with desirable properties such as insect resistance．The next step is to find stretches of DNA that show the presence of a particular gene．Then, when you mate two trees with different desirable properties, it is to check which offspring contain them all by looking for the genetic markers．Henry Amerson, at North Carolina State University, is using genetic markers to breed fungal resistance into southern pines．Billions of these are grown across America for pulp (纸浆) and paper, and outbreaks of disease are expensive．But not all individual trees are susceptible．Dr．Amerson's group has found markers that distinguish fungus-resistant stock from disease-prone trees．Using traditional breeding techniques, they are introducing the resistance genes into pines on test sites in America．
Using genetic markers speeds up old-fashioned breeding methods because you no longer have to wait for the tree to grow up to see if it has the desired traits．But it is more a sophisticated form．of selective breeding．Now, however, interest in genetic tinkering (基因修补) is also gaining ground．To this end, Dr．Amerson and his colleagues are taking part in the Pine Gene Discovery Project, an initiative to identify and sequence the 50,000-odd genes in the pine tree's genome．Knowing which gene does what should make it easier to know what to alter.
Compared with genetic engineering of food crops, genetic engineering of trees______.