Histopathology of Seed-Borne Infections - Applied Research Center ...

Structure of Seeds 63Chalaza simple; only outer one to four layers persist in nucellus, cells in outermostlayer regular and covered by a thick cuticle.Embryo: Cotyledons two, large, thick, fleshy, partially or completely investingthe radicle, cotyledons multi-layered, hypodermis on adaxial surface palisade-like;fats and lipids and protein major reserve food material, procambial strands distinctin cotyledons.Endosperm: One or two layers of greatly compressed cells.3.4.6 APIACEAE (UMBELLIFERAE) (FIGURE 3.9 A TO H)(Gupta, 1964; Sehgal, 1965; Arora, 1976)Fruit dry, schizocarpic, splitting longitudinally into two parts, called mericarps(seed for agriculture). Most mericarps are flattened or concave on one side wherethey remain attached to the carpophore. In cross section, mericarps are asymmetricalwith ridges and furrows. Pericarp forms the protective covering.External: Seed (mericarp) elongate, narrowly oblong, planoconvex, flattened orplano or convexo-concave; usually five or more distinct or indistinct ridges, inDaucus carota and Cuminum cyminum, the primary ridges are intercalated by secondaryridges, which are in the form of multicellular emergences (vallecular ridgesof Heywood and Dakshini, 1971); dark lines between ridges mark the position ofoil ducts (vittae); surface with hairs, prickles, and warts; mericarp scar basal inconspicuous;stylopodium (lower swollen part of style) and calyx (sepals) usuallypersistent at the tip of mericarp.Internal: Mericarp coat (pericarp) thick, differentiated into exocarp (outer epidermisor one or two subepidermal layers), mesocarp (middle layers) and endocarp(inner epidermis); epidermis covered with thick cuticle, stomata present, cells mostlyrectangular, outer wall thick; mesocarp of two or three zones, outer zone mostlychlorenchymatous (Daucus, Cumin, and Foeniculum), sometimes parenchymatous;inner pericarp always parenchymatous; in some cases, e.g., Coriandrum, the cellsof middle layers in mesocarp are tangentially elongated, thin-walled, lignified forminga fibrous zone; endocarp indistinct or distinct of tangentially or radially elongatedthin or thick-walled cells.Mericarps in mesocarp have five vascular bundles in ridges (one dorsal, twolateral, and two commisural bundles); alternate to vascular bundles, vittae occur infurrows. The vittae lie above the vascular bundles in Coriandrum. In Daucus vittaeoccur in both alternating vascular bundles and above the bundles.The carpophore, an area common to the mericarps, is supplied by the ventralvascular bundle. It is flanked by two vittae. At maturity its cells become thick-walledand lignified. Finally it detaches from the tissue of the septum and forms the axison which the two mericarps hang.Seed Coat: Membranous, of more or less crushed cells or with persistent thinwalledouter epidermis.Embryo: Small, basal, rudimentary or axile with two cotyledons.Endosperm: Abundant, cell walls thick, reserve food as oil.