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Primary and Secondary Thickening Meristems
In monocots, which lack a vascular cambium, increase in stem diameter is typically relatively limited. However, most monocots and a few other thick-stemmed angiosperms, especially species with short internodes and crowded leaves, possess a primary thickening meristem (PTM) near the vegetative shoot apex. The PTM (Fig. 2.13) is situated in the pericyclic region. It consists of a narrow multiseriate zone of meristematic cells that produces radial derivatives, usually a limited amount of parenchyma towards the outside (centrifugally), and both parenchyma and discrete vascular bundles towards the inside (centripetally). In addition to primary stem thickening, the PTM is responsible for formation of linkages between root, stem and leaf vasculature. Also, it frequently retains meristematic potential further down the stem and is the site of adventitious root production in some species.
The PTM normally ceases activity at a short distance behind the apex, and subsequent stem thickening is limited. Tree-forming palms possess an extensive PTM that forms a large sunken apex; considerable further stem thickening occurs by subsequent division and enlargement of ground parenchyma cells. This is termed diffuse secondary growth.
In some woody monocots in the order Asparagales (e.g. Agave, Aloe, Cordyline, Yucca) further increase in stem thickness is achieved by means of a secondary thickening meristem (STM) (Fig. 2.14). The STM is essentially similar to the PTM in that it is located in the pericyclic region and produces radial derivatives. However, it is active further from the primary apex and produces second-ary vascular bundles that are often amphivasal and radially elongated. In some species (e.g. Nolina recurvata, Cordyline terminalis) the PTM and STM are axially discontinuous104,105, whereas in others (e.g. Yucca whipplei) they are axially continuous26. Apart from the distance from the apex, there are no precise criteria for distinguishing between derivatives of the two meristems, and transitional forms exist. Thus, they are perhaps best regarded as developmental phases of the same meristem.
The PTM and STM are not homologous with the vascular cambium, because the vascular derivatives are arranged in different ways. The vascular cambium produces phloem centri-fugally and xylem centripetally, whereas most derivatives of the PTM and STM are centripetal, and consist of a parenchyma-tous ground tissue and discrete vascular bundles containing both xylem and phloem. Furthermore, the PTM originates in ground tissue, is a tiered meristem, and is often fairly diffuse, especially near the shoot apex (Fig. 2.13). By contrast the vascular cambium is typically uniseriate and initially originates within vascular tissue, though it later extends between bundles.
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