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

Structure of Seeds 53dry seed of Lycopersicon and some Solanum species are not true hairs. Saxena (1970)has shown that these are the radial walls of seed epidermal cells with thickeningdecreasing from inside to outside. These have been termed spurious hairs by Corner(1976) and pseudohairs by Rick (1978).Seed appendages do not occur universally and they are rare in crop plants. Theycause an increase in seed surface and this may promote association of microorganisms;however, no data exist to support this fact.3.3 INTERNAL MORPHOLOGY3.3.1 GROSS INTERNAL MORPHOLOGYThe gross internal morphological features of the main components of seed, i.e., theseed coat, endosperm and perisperm, and embryo, give an idea of their spatial andtopographic adjustments. The usefulness of these features as taxonomic pointers iswidely recognized (Bouman, 1974). These features are also significant in the functionalperformance of seed.Martin (1946) described the comparative morphology of seeds of 1287 generaof angiosperms and proposed a classification of seed types on the basis of the sizeof the embryo in relation to the endosperm, and on the differences in size, shape,and position of the embryo (Figure 3.3). He used embryo measurements in quarterunits of a circle and designated five types: (1) small, with the embryo smaller thana quarter of internal space; (2) quarter, with a quarter or more but less than twoquarters internal space; (3) half, with two quarters or more but less than three quartersinternal space; (4) dominant, with more than three quarters internal space; and (5)total, with the embryo occupying the entire inner seed space.On the basis of embryo position, Martin (1946) proposed three main categoriesof seed: basal, peripheral, and axile. Because of their overlapping features the basaland peripheral categories are merged into a single category called the peripheral.These categories are further subdivided on the basis of size and shape of the embryo.The peripheral type with the embryo at the micropylar end or oriented peripherallyand with copious endosperm or perisperm has five subcategories: (1) rudimentary,where the embryo is small, globular to ovate oblong, and relatively undifferentiated(Magnolia and Piper); (2) broad, where the embryo is as wide as or wider thanlong, globular, or lenticular (Nymphaea and Juncus); (3) capitate, where theembryo is distally expanded (Dioscorea, Tradescantia, Scirpus, and Carex); and(4) lateral, where the embryo is lateral, inclined to expand along the periphery(Poaceae); and (5) peripheral, where the embryo is elongated, large, often curved,extending along the periphery, with cotyledons narrow or expanded (Amaranthaceae,Chenopodiaceae).The axile type has small to large embryo, straight, variously curved or coiled,central, seeds small to large, and endospermic or nonendospermic. It has sevensubtypes: (1) linear, where the embryo is much longer than broad, with the cotyledonsstraight, curved, or coiled (Lilium and Allium); (2 and 3) dwarf and micro,where the seeds are small, with the seed interior 0.2 to 0.3 mm long in (2) and lessthan 0.2 mm long in (3), the embryo is small or large (Orchidaceae, Burmanniaceae,