The Functional Differentiation of the Testa in Seed of Indigofera parviflora (Leguminosae: Papilionoideae)

Abstract

The mature ovule of Indigofera parviflora has two three-seriate integuments. The outer integument participates in testa formation; the inner is crushed during seed development. The strophiole is distinguishable at an early stage and differentiates rapidly. Cells of the outer epidermis (O1) divide anticlinally and elongate radially to form a palisade layer. The outer tangential walls are thickened at an early stage, and deposition of fluted thickenings on the radial walls occurs at maturity. Palisade cells at the hilum differentiate from subfunicular tissue and are indistinguishable from the epidermal palisade at maturity but differentiate more rapidly. Differentiation of the tracheid bar coincides with final secondary wall deposition without lignification in the palisade. Differentiation of a layer of osteosclereids from the hypodermal layer (O2) accompanies palisade differentiation. The inner epidermis (O3) is crushed during seed development. The raphe cells at the strophiole are shorter and differentiate sooner than the accompanying cells and are bowed under tension. They maintain distal continuity by early development of a very thick outer tangential wall. The strophiolar hypodermis cells elongate greatly, raising the strophiole. The walls of the strophiolar hypodermis are electron translucent and consist of a loose fibrillar network embedded in an amorphous matrix. The functional significance of this unusual wall structure is discussed: water enters the seed at the strophiole, and controlled imbibition appears to be achieved passively through the structure of the hypodermis.