blr , bp and rev mutant xylem. Xylem from vascular bundle in inflorescence stems. (A) Wild type xylem with vessels (xv) and xylary fibres (xf) marked. (B) bp mutant xylem is indistinguishable from wild type (left hand side; LHS), except in differentiation defective tissue (right hand side; RHS) where vessels are absent or reduced in size and many cells lack secondary walls (green in wild type xylem; A). (C) Extreme blr phenotype where xylem vessels appear absent. (D) blr bp xylem in later derived tissue (see Figure 5F) with xyary fibres but no vessels (LHS). In differentiation defective regions of the stem (RHS) Few cells with secondary wall are present. (E) rev xylem, similar to wild type. (F) bp rev xylem is like that of rev (LHS) except in differentiation defective regions (RHS) where cells were smaller than those observed in bp. (G) rev blr xylem. (H) Most blr bp rev xylem lacked secondary walls. Scales are 10 μm.