THE GROWTH OF THE CELL-WALL 27 



SECTION 8. The Growth of the Cell-wall. 



Every increase of size or external change of shape in a typical 

 plant protoplast involves a corresponding alteration in the shape or surface 

 extent of the investing cellulose membrane. After the stretching-growth 

 of the cell has ceased, the cell-wall commonly undergoes secondary 

 thickening, which is definitely localized to form pitted, annular, spiral, 

 or reticulate deposits. In addition physical and chemical changes may 

 occur, and all of these processes are carried out and directed by the living 

 protoplasm. The latter also provides the material for the growth of the 

 cell-wall, which cannot take place unless direct contact is assured between 

 the ectoplasm and the cell-wall. 



As is usually the case, the same end may be attained in a variety of 

 ways. Thus the newly formed cell-wall is not always composed of the 

 same materials, and in certain cases it may be built up of secreted products, 

 but in others by the direct metamorphosis of particles of the peripheral 

 protoplasm. Similarly it is certain that the subsequent growth may take 

 place either by apposition or by intussusception, and as far as our know- 

 ledge goes the secondary thickening usually, though not always, takes 

 place by apposition. On the other hand, intussusception, together with the 

 stretching of the plastic wall, is probably mainly responsible for the 

 growth in surface extent. Growth by apposition combined with stretching 

 will also keep a growing cell-wall of constant thickness. 



The energy for growth by intussusception (active growth) is deter- 

 mined by the molecular processes involved in the penetration of the 

 interpolated particles, and upon the power of swelling possessed by these. 

 Growth by stretching is on the other hand the result of the osmotic 

 pressure within the cell, and hence such passive growth is impossible 

 unless this pressure is sufficiently great. The energy involved in growth 

 by intussusception has a much higher mechanical intensity than osmotic 

 energy, and moreover the latter must be overcome when, by the secondary 

 growth in thickness of the cell-wall, the cavity of the cell is diminished. 



Owing to the power of self-regulation possessed by the organism, a grow- 

 ing cell may temporarily retard or inhibit its growth when necessary. The 

 same occurs in aerobic plants in the absence of oxygen, owing either to the 

 processes leading to intussusception not being performed, or in the case of 

 stretching-growth owing to the necessary diminution in the cohesion of the 

 cell-wall not taking place. The osmotic pressure is usually unable to per- 

 manently stretch the cell-wall in the absence of any such plastic softening, 

 for under ordinary circumstances it is not stretched to its limit of elasticity. 

 This is shown by the fact that the tension in the walls may be increased by 

 one-third without any permanent stretching, if weights are attached to the 



