grouping questions

Hi, 

I have some problems in restructuring some data files. what i have to do is
to group the measurements(<length>) of "a" and "c" (<Axis>) together, so i
need to find what are the other tags in each row, and read their values to
decide which ones belong to the same experiment. 

my xml file looks like this:

<Record sample="1">
   <row row_id='0'>
      <length>5.4</length>
      <Axis>a</Axis>
      <Environment>air</Environment>
      <Temperature>200K</Temperature>
   </row>
   <row row_id='1'>
      <length>5.3</length>
      <Axis>c</Axis>
      <Environment>air</Environment>
      <Temperature>200K</Temperature>
   </row>
   <row row_id='2'>
      <length>5.0</length>
      <Axis>a</Axis>
      <Environment>air</Environment>
      <Temperature>400K</Temperature>
   </row>
   <row row_id='3'>
      <length>5.4</length>
      <Axis>c</Axis>
      <Environment>air</Environment>
      <Temperature>400K</Temperature>
   </row>
   <row row_id='4'>
      <length>5.4</length>
      <Axis>a</Axis>
      <Environment>Ar</Environment>
      <Temperature>200K</Temperature>
   </row>
   <row row_id='5'>
      <length>5.4</length>
      <Axis>c</Axis>
      <Environment>Ar</Environment>
      <Temperature>200K</Temperature>
   </row>
</Record>
<Record sample="2">
        ...
</Record>

Problem is, tags other than <length> and <Axis> vary from sample to sample.
this seems like impossible for me to do the grouping using xslt. Maybe some
gurus here can give me your brilliant ideas?... thanks!!

Xiang