The genesis of intercellular spaces in developing leaves of Phaseolus vulgaris L.

Observations by light, transmission electron and scanning electron microscopy have shown that intercellular spaces (ICS) are formed schizogenously in expanding leaves of Phaseolus vulgaris. ICS formation occurs in predictable positions at the junctions between three or more cells, and follows three phases of development. The first, initiation, phase occurs soon after cell division, and is marked by the formation of an electron-dense osmiophilic body, probably proteinaceous, at the end of the cell plate/middle lamella of the daughter cell wall and across the adjacent piece of the primary wall of the mother cell. This part of the mother cell wall is digested, involving cellulolysis. The second phase, of cell separation, is marked by the first appearance of the ICS. In Phaseolus primary leaves this phase begins about day 3 after sowing, at which time the leaf area is about 1 cm². In the final enlargement phase, lysis of cell wall material continues in the region of the middle lamella, and mechanical tensions arising from the rapid expansion of the lamina lead to further separation of the mesophyll cells so that spaces enlarge and merge.