A seco ndary thickening meristem is record ed for the first time in some herbaceous taxa of Asparagales (Herreria montevide nsis and Thysanotus sp inige r), and the new records are assessed in a systematic co ntext. All monocotyl edons lack a vascular cambium, which is typicall y a single per sistent row of cells producing phloem centrifugally and xylem ce ntripetally. However, some monocotyled ons achieve stem thicken ing by means of a different type of later al meristern, either a primary thickening meristem (PTM) near the apex, together with diffuse secondary growth (as in palm s). or a secondary thicken ing meristern (STM) further away from the apex (as in some Asparagales; see Rudall 1991, for review). The PTM and STM are probabl y developmenlally related, although there is complex tissue involvement. In taxa with an STM, the PTM and STM may someti mes be longitudi nally co ntinuous, at least at some stage in the life cycle (Steven son 1980). Virtually all monocotyledons have a PTM , but among tree-forming or woody taxa this has developed along different lines, probabl y more than once, either as an exten sive apical PTM, as in palms, or as an STM, in some Asparagales (see below). The PTM and STM are not homologous with the vascular cambium, as they are tiered (etagen) men stems which produce distinct vascular bundles (of both xylem and phloem) in a parench ymatou s ground tis sue (Fig. 1-3), mainly ce ntripe tally. There are record s of a PTM in some dicotyledon s and other plant groups (DeMaso n 1983), although the hom ology of these requ ires testing . All unequi vocal records of an STM are in the asparagoid clade (sensn Cha se et al. 1995), which compri ses Dahlgren et al.'s (1985) order Asparagales, together with a few membe rs of their Liliales, such as Iridaceae (Rudall 1991). A STM has been re ported in several tree-forming and shru bby asparagoid s: Aga ve, Aloe, Beaucarnea, Calibanus, Cordyiine, Dasylirion, Dra caena, Furcra ea, Klatti a, Nive nia, No lina, Pleomel e, Sansevieria, Witsenia, Xanth orrh oea and Yucca, and also in some more her baceou s taxa with a thick ' woody' rhizome or stem, such as Aphyllanthes, Gasteria.