CYTOLOGY 13 



are much too great for this to be possible. It is due either to the inter- 

 calation of new sets of fibrils between those already existing or to their growth 

 in length by the addition of new glucose molecules. Growth by this inter- 

 calation process is called intussusception and is characteristic of the young 

 phase of the cell, while the addition of successive thickening layers, con- 

 centrically, is called apposition and is a feature of cell maturity. 



Secondary cell walls often show a visible structure of concentric layers. 

 These are not, of course, single sheets of cellulose molecules, but seem to 

 represent variations in the amount of cellulosic and non-cellulosic material 





Nucleus 



Fig. 2. — Tradescantiavirgimava. Cells of a staminal 

 hair, showing nuclei and strands of cytoplasm in 

 which streaming movements take place. The 

 vacuole contains a solution of a purple pigment. 



present. In the wall of the cotton fibre it has been possible to correlate these 

 concentric layers with the daily metabolic cycle, the denser layers representing 

 deposition by day, and the lighter layers being formed during the night. 

 They are analogous to the growth rings in trees, and are similarly affected 

 by variations in the environment at different times. They can be definitely 

 dated, and the corresponding layers in the cell walls in neighbouring plants 

 are found to be similar. 



Cytoplasm 



The living cytoplasm, where it can be observed in uninjured cells, appears 

 as a colourless, viscous fluid, which frequently shows streaming movements 

 or a circulation round the cell, called cyclosis (Fig. 2). This striking move- 

 ment of the living substance has been the subject of a great deal of investiga- 

 tion but no final explanation has emerged. The energy is certainly provided 

 by respiration, and the most probable view of the mechanism is that it is due 

 to rhythmic contraction and relaxation of the protoplasmic structure. 



