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

3Structure of SeedsThe seed habit is a significant advancement in the evolutionary history in the plantkingdom and has bestowed several advantages to seed-bearing plants. Biologically,the seed is the ripened ovule, as discussed in Chapter 2. However, the use of theterm seed is not always restricted to this morphologically accurate definition. Seedis usually applied in the functional sense, i.e., as a unit of dissemination, a disseminule.In agriculture, in addition to true seeds, many one-seeded dry indehiscentfruits, namely caryopsis (Poaceae), cypsil (Asteraceae), cremocarp and mericarp(Apiaceae), and achene and utricle (Amaranthaceae and Chenopodiaceae), aretermed seeds. The term seed is used sensu lato in this volume.Seed structure is intimately concerned with the infection caused by plant microorganisms.Seed morphology and anatomy reveal the probable pathways of infectionas well as the barriers to infection; therefore, we include a detailed general accountof the structure of seed in angiosperms as well as brief specific accounts of seedsof 12 plant families that contribute to the majority of crop plants.3.1 CONSTITUTION OF SEEDSA viable seed consists of an embryo, a protective covering, and the reserve foodmaterial. The embryo comprises one or two (and rarely more) cotyledons, a plumulebud (epicotyl), a hypocotyl, and a radicle. The embryo varies in size and shape. Theembryo is undifferentiated in many plants, particularly parasitic, saprophytic, andinsectivorous plants. The cotyledons are absent in such cases. The embryo is narrow,elongated, and spirally coiled with plumular scales in Cuscuta reflexa (Johri andTiagi, 1952).The protective covering usually comprises a seed coat in true seeds, and apericarp and a seed coat or a pericarp in one-seeded fruits (Apiaceae, Asteraceae,and Poaceae), bracts in Oryza and Hordeum, a calyx in Brunnonia, and a cuticle ofendosperm in Crinum, Santalaceae, Loranthaceae, and Olacaceae. There is considerablevariability in the anatomy of the seed coat and pericarp.The reserve food material is stored in the endosperm (seeds called endospermic),the perisperm (persistent nucellar tissue), or the embryo (cotyledons), or in both theembryo and the endosperm. The major reserve foods are carbohydrates, fats andlipids, and proteins. Seeds also contain other minor reserves, some of which arenutritionally undesirable or even toxic, e.g., alkaloids, inhibitors, lectins, and phytins.The cuticula of different components of seed are distinct. The surface of eachcomponent is covered by a thick or thin cuticle.47