When plants are being examined for comparative purposes, either for identification or for taxonomic reasons, it is important that similar parts of the organs are looked at. The leaf, for example, is normally looked at in trans-verse section across its broadest region, or halfway along the length of the lamina. The surface of the leaf is studied near the central region of the lami-na. The margin may also be examined.
Petioles should be examined in transverse section just where the lamina begins, halfway down its length and also near the base. Stems are normally sectioned in the middle of the internode, or, in addition, at the node (Fig. 10.7).
Roots are normally sectioned at a convenient level, since accurately de-fined positions are harder to delimit. For very detailed studies, of course, sections are required from many other levels, and sometimes serial sections are needed. These are particularly useful in the study of nodes and shoot apices.