In mature tissues, spaces frequently occur between cells. These spaces are known as intercellular spaces. The spaces may be small, large (ex. Canna)—called chambers and elongated—called canals. The spaces may be considerably elongated in one plane—called duct.
The spaces are exceptionally well developed in aerenchyma of many aquatic plants. They form a connected system throughout the entire plant body. The spaces may be vertical and are filled with air and water, thus forming air spaces and air canals.
Air spaces and air canals help in aeration and add buoyancy to plant. It has been suggested that, the honeycomb-like system of intercellular spaces in aquatic plants can withstand considerable mechanical stress.
In terrestrial plants, the enlarged intercellular space gives rise to secretory glands (ex. Citrus sp.) or ducts (ex. Pinus).
Parenchyma of the endosperm of most seeds is devoid, or almost so, of intercellular spaces. These cells are very tightly packed and are in close contact all round. Meristematic tissues are usually thought to be devoid of intercellular spaces. Observation with electron microscope, on the root tip of Allium, reveals the presence of intercellular space of 20 µm. Such a small space could not be readily observed with the aid of light microscope.
According to the mode of development, there are two types of space, schizogenous and lysigenous. The former is most common and results from the separation of cell walls from each other along certain areas of their contact. The word schizogenous is derived from the Greek word-schizo meaning split and genesis meaning beginning.
Previously it was thought that these spaces result from the splitting of the middle lamella and so the space are called schizogenous. The schizogenous development of an intercellular space is described as follows. At the end of cell division a cell plate is formed separating the two sister protoplasts.
The plate forms the middle lamella. The components of the cell wall then deposit on both the sides of middle lamella to form primary cell wall. The newly formed middle lamella makes a contact with the primary wall inside of the parent cell at the sides only and not with the middle lamella, that cements the parent cell with neighbours.
At the point of contact between the new middle lamella and primary wall of parent cell a small space develops. This space enlarges due to the dissolution of primary wall of parent cell at opposite side of space. This space is more or less triangular in outline. The components of middle lamella deposit on the sides of triangular space.
The new middle lamella becomes continuous with the recent deposits. Therefore, the unit structure appears to be forked. The forked portion gradually extends because of growth of daughter cells and ultimately joins with the middle lamella of parent. Thus, an intercellular space is formed within the forked middle lamella (Fig. 3.21).
Diagrammatic representation of the stages in the formation of intercellular space. 1. Dividing parent cell at the end of telophase. 2. Newly formed middle lamella in contact with the primary wall of parent cell. 3. Appearance of space. 4. Newly formed middle lamella in contact with the middle lamella of parent cell.
The parent cell may have similar space with the neighbour. This old space may join with the new one to form a large intercellular space. In schizogenous development of spaces, a portion of parent cell wall is dissolved only. These newly formed cells enlarge in all directions to assume the normal size.
The cell walls separate and the schizogenous space is the result. Meristematic cells are tightly packed, and during differentiation to parenchyma, schizogenous spaces develop. Examples of this schizogenous cavities or ducts are: resin ducts of Pinus and secretory ducts of Asteraceae and Apiaceae, secretory cavities of Eucalyptus etc.
The lysigenous intercellular spaces are formed by the disintegration of entire cells. The term lysigenous is derived from the Greek word—lysis meaning loosening and genesis meaning beginning.
The large air spaces of water plants, air spaces in Zea root, primary resin duct of Mangifera indica etc. are the examples of lysigenous spaces. The protoxylem lacunae of Zea mays is formed by both schizo- and lysigenous way. Such spaces are called schizolysigenous.