apical meristem



apical meristem defined in 1951 year

apical meristem - apical meristem;
apical meristem - Growing point (zone of cell division) at tip of root and stem in vascular plants, having its origin in a single cell (initial), e.g. Pteridophyta, or in a group of cells (initials), e.g. Spermatophyta. In the latter the apex of the growing point (promeristem) consists of actively dividing cells. Behind the pro-meristem division continues and differentiation begins, becoming progressively greater towards the mature tissues. One (older) concept of the organization of the growing point in flowering plants recognizes differentiation into three regions (histogens), dermatogen, a superficial layer of cells giving rise to the epidermis, plerome, a central core of tissue that gives rise to the vascular cylinder and pith, and periblem, tissue lying between dermatogen and plerome, that gives rise to cortex. It is now evident that the respective roles assigned to these histogens are by no means universal, nor is it always possible to distinguish between periblem and plerome, especially in the shoot apex. Now becoming widely accepted is the Tunica-Corpus concept, an interpretation of the shoot apex which recognizes two tissue zones in the promeristem, tunica, consisting of one or more peripheral layers, in which the planes of cell division are predominantly anticlinal, enclosing corpus, or central tissue of irregularly arranged cells in which the planes of cell division vary. No relation is implied between cells of these two regions and differentiated tissue behind apex as in histogen concept. Although epidermis arises from outermost tunica layer, underlying tissue may originate in tunica or in corpus, or in both, in different plant species.

In addition to providing for growth in length of main axis, apical meristem of stem is the site of origin of primordia of leaves and buds. In roots two types of apical meristem occur, one in which vascular cylinder, cortex and root cap can be traced to distinct layers of cells in the promeristem and a second type in which all tissues have a common origin in one group of promeristem cells. In contrast to that of stem, apical meristem of roots provides only for growth in length, lateral roots originating some distance from apex, and endogenously, from pericycle.

near apical meristem in Knolik


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aplanospore

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