Fig. 15.3-11a and b. Transverse section of magnolia wood (Magnolia tripetala). In this wood, most vessels occur in clusters: select several clusters and count the number of vessels in each. Only count a vessel as being a member of the cluster if it actually touches another vessel of that cluster; if it is separated from the vessels of a cluster by a thin layer of fibers (or a layer of axial parenchyma in those species that have paratracheal parenchyma), then it is not a member of the cluster. Typically, at least a few vessels occur as solitary vessels, even in woods where the great majority of vessels are clustered.