Growth of the cell wall-digieduco

Cell wall grows in surface as well as in thickness. Due to the stretching of thin, soft and elastic primary cell wall the cell increases in size. In a young cell, the wall is thin and delicate ; as the cell grows and becomes mature, the growth in thickness of the cell wall begins. there are two classical theories regarding the growth of the cell wall in thickness, such as : (a) that growth by intussusception method, where the new cell wall materials i.e. new microbfibrils are held to be laid down between the existing microfibrils of the expanding cell wall, and (b) that of growth by apposition method where new microfibrils are laid down on top of the existing ones forming a new layer. Though there are two theories, it is now considered that the formation of both primary and secondary cell walls occurs mainly by the apposition method (Muhlethaler, I961). It is probable, however, that some growth in surface area as well as in thickness of the cell wall by intussusception method does occur. Growth by apposition is generally centripetal i.e. takes place from outside to the middle. 

Diagram showing growth of the cell wall by (a) intussusception (b) and apposition.

With respect to longitudinal growth of the cell wall, a new theory known as multi-net theory of the cell wall growth has been accepted widely (Muhlethaler, 1961, 1965 ; Wardrop and Harada, 1965 ; Preston, 1974 ; Deshpande, 1976). This theory also accounts for the observed orientation of the microfibrils in successive layers of the wall. According to this theory microfibrils are first deposited more or less transversely to the longitudinal axis of the cell, and this layer is later pushed outwards due to the formation of another layer internal to it by apposition method. As the cell elongates, the first fonned layers of microfibrils are stretched and thus become oriented in a progressively more longitudinal plane. Frey-Wyssling and Stecher (1951) suggested that the growth of the primary cell wall takes place in a way that has been termed mosaic growth. In this type of growth, thin areas penetrated by the cytoplasm appear in the growing primary wall. In these regions synthesis of cytoplasm takes place, as a result the microfibrils are pushed apart by the enlarging mass of cytoplasm, thus enlarging the cell surface. Again new microfibrils are woven into the wall to fill in the thinner areas.