THE PRIMARY THICKENING MERISTEM: DEFINITION AND FUNCTION IN MONOCOTYLEDONS

The PTM should be defined as a diffuse primary meristem which decreases in cross‐sectional extent (i.e., becomes a thinner‐walled cylinder) in a basipetal direction. It is associated with extensive anticlinal cell files and consists of cell initials that divide predominantly in periclinal planes. This meristem occurs typically in monocotyledons, especially those with thick, compact stems in species with rosette shoot axes. The PTM is also associated with a wide crown, so that the apical meristem is either slightly above the level of youngest leaf primordia, at approximately the same level as the leaf primordia, or distinctly sunken below surrounding stem tissue and the youngest leaf primordia. The location is dependent on the extent of primary thickening growth occurring in a particular species. A meristem associated with primary thickening of other plant groups should not be called a primary thickening meristem unless all of the above characteristics are shown to be associated with the meristem being examined. The primary thickening meristem is responsible for primary thickening of a stem axis. Its ontogenetic relationship with the STM needs further investigation. Extensive primary stem thickening has been observed in non‐monocotyledons (ferns, lycopods, cycads, and dictyledons). Some of these organisms appear to undergo primary thickening from a PTM in a similar process as that which occurs in monocotyledons. Further research is necessary to establish the mechanisms of primary thickening in these cases.