Microfibrillar structure of growing cell wall in a coenocytic green alga,Boergesenia forbesii

A fine structure of cell wall lamellae in a coenocytic green algaBoergesenia forbesii was examined by electron microscopy. The wall has a polylamellate structure containing cellulose microfibrils 25 to 30 nm in diameter. The outer surface of the cell was covered by a thin structureless lamella, underneath which existed a lamella containing randomly-oriented microfibrils. The major part of the wall consisted of two types of lamellae, multifibrillar lamella and a transitional, matrix-rich one. In the former, microfibrils were densely arranged more or less parallel with each other. In the transitional lamella, existing between the multifibrillar ones, the microfibril orientation shifted about 30° within the layer. The fibril orientation also shifted 30° between adjacent transitional and multifibrillar layers, and consequently the microfibril orientation in the neighboring multifibrillar layers shifted 90°. It was concluded that the orientation rotated counterclockwise when observed from inside the cell. Each lamella in the thallus wall become thinner with cell expansion, but no reorientation of microfibrils in the outer old layers was observed. In the rhizoid, the outer lamellae sloughed off with the tip growth.