27, 30, 31), or forming a conspicuous tissue between and partly enclosing the two parts of the bundle (figs. 28, 29).

The leaf-section furnishes sectional and other lesser distinctions. It is often decisive in separating species otherwise difficult to distinguish (nigra and resinosa or Thunbergii and sinensis, etc.). Sometimes it is sufficiently distinct to determine a species without recourse to other characters (tropicalis, oocarpa, Merkusii, etc.). An intimate knowledge of the leaf-section, with an understanding of the limits of its variation, is a valuable equipment for recognizing species.