Bob raises a number of important questions. I'll try to make my two cents worth clear on a couple of them.

    First, he mentions a sixth grade text that had two appositives. He also notes that the text "was at times incoherent because of the minimal amount of subordination." That doesn't surprise me. Over a decade ago I did a small research project in which I analyzed the writing of third, fifth (I think), seventh, eighth, and ninth graders. I also analyzed passages from the English and science texts these students were supposed to use. The textbooks were extremely heavy on gerundives and appositives, and, if I remember correctly, light on subordinate clauses. (In other words, I think I saw the same kind of thing that Bob is talking about.)

     The source of the writing samples also gave me test scores of the students. One of the things that particularly interested me was that the two students with the lowest test scores were also the most frequent users of gerundives (participles). I have to wonder if these two students were highly influenced by the textbooks, and influenced in the absence of reading at home. (As someone said ― and I agree ― the best way to master grammar is to read, read, read.) I am suggesting, in other words, that the textbooks may have CAUSED the students' problems.

     I too will be interested in Bob's summary of Perera, but I probably will not trust his (Perera's) conclusions. Remember, I have studied all that research that "proved" that grammar is harmful, etc., and I have seen how research can be twisted. What I want to see are more samples of students' writing. If Perera provides all the samples of writing that he has studied, I'll be happy. (I am, by the way, practicing what I preach. My research ― and the students' writing ― are at:

http://www.pct.edu/courses/evavra/ED498/R/Spr95/index.htm

Anyone who wants to can look at the data and question and/or challenge my conclusions. Bob, for example raises the question of whether the appositives are in the subject or object. I agree that that is a very important question, but I also want to know exactly what is counted as an appositive. If the student writing is available for all to examine, then, I think, we could make real progress.

      Would anyone be interested in the samples, etc. from the project I mentioned above? If so, I think I have it somewhere still on computer, and I will try to post it too on the web.

    Bob raises another important question when he wonders what "to teach appositives" means. It's a very interesting and complex question. I am, for example, hesitant about any instruction that is directly designed to get students to use a specific construction. When my son was in third or fourth grade, he brought home an assignment in which he was expected to combine "Mary studies animals." and "Mary is a biologist." into Mary, a biologist, studies animals." Not only did he have no idea of what was going on, he was very frustrated. Later, I read Hunt's article on "Late Blooming Constructions." Hunt suggests that appositives, as I think I already noted in the last post, develop after mastery of subordinate clauses. And, as Bob just suggested, my third or fourth grader was expected to use a front-loaded appositive. 

     Note that I am not saying that I oppose any instruction which attempts to get students to use constructions. I would have no trouble with ninth graders being asked to use subordinate clauses. In general, however, I think we should teach students to recognize syntactic constructions and then have them discuss their use. When my college Freshmen have learned to recognize subordinate clauses, have analyzed a passage of their own writing for them, and then worked in groups to review and discuss their analysis, the students who used few (or no) subordinate clauses WANT to use more. Those students with several triple-embeddings (by which I mean a subordinate clause within a subordinate clause within a subordinate clause) realize that they are on the heavy end and might want to lighten up. 
     To me, our job is to help students understand how sentences work, how various constructions affect style, etc., but it is up to the student to decide what they do and do not want to use.
     Enough for today. It's 3:46 on Friday afternoon.