STRUCTURAL COMPOSITION OF THE SCLEREIDS IN THE INTEGUMENT OF PISUM SATIVUM L.

PROBLEMS OF texture constitute a major field of technological investigations in the commercial processing of vegetables as food. The causes of texture change in the seed coats of beans and peas intended for freezing or canning not only involve biological variables but also the effects of harvesting, handling, and processing treatments. Food research has been confined mainly to these latter environmental effects on the qualities of the processed products. Existing information on cell-wall composition and other histological factors of texture has been either incomplete or unreliable, particularly with respect to the chemical nature of the secondary wall structure in the epidermal cells of the seed coat. The histochemical studies reported here were undertaken in order to clarify confusing information and to establish facts on the compositional nature of the tissues responsible for seed-coat texture. The results of these studies are of direct interest to plant anatomy and should provide significant and useful information to applied research in commercial food preservation. ANATOMY OF THE INTEGUMENT. Structure of the epidermis and hypodermis.-Many detailed accounts have been written about the anatomy of the ovule and integument in leguminous species (Pammel, 1899; Netolitzsky, 1926; Hamly, 1932-1935; Zimmerman, 1936; and others). Therefore, it should be necessary to present only a general description of the cellular organization (fig. 1, 2, 3). In beanis and peas the epidermis is highly specialized at maturity into elongated cells with thickened walls and possesses a well-defined cuticle. These cells have been variously designated as Malpighian cells, prism cells, palisade cells, and macrosclereids. Underlying them is a hypodermal layer ciomposed of specialized cells which have been called "I" cells, or "hourglass" cells. The term "Tragerzellen," used by Zimmerman (1936) and previous investigators, is well suited because of their obvious resemblance to pillars supporting the epidermis (fig. 3). The terms macrosclereids and osteosclereids are used here for the cells of the epidermis and hypodermis, respectively, because their histological classification is considered more desirable than descriptive reference to their appearance (see Foster, 1942, on tissue classification and chapter on sclereids where Tschirch is cited). The macrosclereids are further characterized at maturity by fluted, twisted wall thickenings which are most pronounced in the apical portions of the cells. These thickenings undergo considerable increase during growth in the harvest season (fig. 4 to 7 and 10 A-B). In the basal third of the cell they are less pronounced, except at the extreme 1 Received for publication November 19, 1945. basal end, and the secondary wall surrounding the lumen is more or less uniform in outline (fig. 6, 7 and 10 A-B). At the extreme basal end of the macrosclereid the lumen is constricted further by a uniform secondary wall thickening which may become more or less pronounced as the integument of the pea seed matures (fig. 3). The much discussed "light line" of this tissue layer may be observed always near the apical ends of the cells, in close proximity to the termination of the lumen and where the fluted thickenings occupy nearly all of the cell cavity and converge to form the end wall. The lumen is extended between the thickenings as minute, finger-like slits which some of the earlier literature describes as "pore canals" (fig. 4, 5). They have no relationship to the primary and secondary wall pits of present-day plant anatomy and are only the result of the crowding or filling of the cell cavity by the specialized wall thickenings. The osteosclereids also possess thickened longitudinal walls, the secondary wall development being most evident along the constricting curvatures of the central portion of the cell. Near the hilum region, and to a lesser extent elsewhere in the integument of the pea, the thickenings may be somewhat fluted and slightly spiraled (fig. 16). In garden varieties of peas and lima beans these cells are somewhat more uniform in shape and structure than in many other leguminous species, in which they display considerable variety in form (Pammel, 1899; Tschirch and Oesterle, 1893; Netolitzsky, 1926; Hamly, 1932; Zimmerman, 1936; and others). Underlying the osteosclereids is a loosely organized parenchyma (fig. 1, 2). In many genera there are further tissue specializations in the integument, and the endosperm may be well defined with aleurone cells (Pammel, 1899, and Zimmerman, 1936). In the common garden varieties of beans and peas these other tissues are ill defined or lacking when the ovule is approaching maturity. The macrosclereids at the hilum region form a double layer under which the osteosclereids are modified into a special zone surrounding a pocket of reticulate, lignified sclerenchyma. Approaching this region, the osteosclereids may be somewhat more elongate than elsewhere in the integument. Along the top of the seed, opposite the hilum and at the strophiole region, the macrosclereids become elongated, frequently two or three times their normal length over most of the seed coat (Zimmerman, 1936). Otherwise the structural characteristics and composition of these two layers are more or less uniform in the integuments of beans and peas. Significance of specialization.-Functioning of the integument in germination of leguminous seeds has