Welwitschia mirabilis: Fine Structure of the Germinating Seed IV. The Imbibed Gametophyte

Abstract

Summary During the first two days after imbibition, in the cells of the gametophyte of Welwitschia, the volume of the nucleolus increases and membranes become clearly defined. Microbodies increase strikingly up to day 3, but decline in number after day 4 as lipid reserves are consumed. Microbodies, often containing spherical inclusions within an amorphous electron dense region near the enclosing membrane, and lipid bodies are sequestered into protein bodies. Digestion of protein body reserves begins from within the protein-carbohydrate deposits, giving the body a fenestrated appearance. Small, spherical, electron dense crystalline inclusions, presumably globoids, are found not only in the protein-carbohydrate deposits of the protein body, but also in contact with the outer protein body membrane as well as in the cytoplasm. This suggests a cytoplasmic origin for the globoid. Mitochondria, ER and ribosomes are numerous by day 4; amyloplasts, dictyosomes and myelin figures appear by day 5. By day 6 protein bodies fuse to form aqueous vacuoles, lipid reserves are depleted and amyloplasts are numerous. Although nuclear inclusions disappear upon imbibition nucleoli tend to remain in a semi-quiescent state; this, coupled with the pattern of protein body degradation, suggests an embryo-controlled mobilization of food reserves.