CYTOLOGY II 



cell as a static object but as a functioning mechanism, and the growth of 

 experimental cytology, including the micro-dissection of cells, has opened 

 roads of the highest promise towards a fuller understanding of life processes. 

 We will now consider the more important cell components in order. 



The Cell Wall 



The primary wall is laid down at cytokinesis or cell-division. In many 

 lower plants, and in the formation of spores in many plants of all grades, 

 the cytoplasm divides by furrowing, that is by the formation of a cleavage 

 plane which develops inwards from the old wall towards the centre of the 

 cell. In the great majority of cell divisions, on the other hand, the two 

 daughter cells are separated by a cell plate, which forms between the two 

 daughter nuclei towards the end of the nuclear division process. The cell 

 plate first appears near the middle of the cell and grows outwards until its 

 edges touch the old wall and join it. 



The cell plate is at first a fluid film, but it is soon solidified by the deposition 

 in it of pectin and protein. The latter soon disappears from new cell walls 

 in mature tissues, but it seems to be more persistent in the cell walls of 

 meristematic tissues. This first solid layer forms the intercellular matrix or 

 middle lamella. Its subsequent changes of composition are not yet clear, 

 but it seems probable that the pectin or pectinic acids first present become 

 combined with cellulose to produce an insoluble substance closely resembling 

 pectose, and that Calcium later enters into its composition, perhaps in the 

 form of Calcium pectate. In woody tissues, however, the middle lamella 

 may become intensely lignified (about 70 per cent.) and pectic substances 

 disappear or are at least masked. 



While the middle lamella is still quite young a thin cellulose layer is 

 deposited on each side of it, and the three layers together constitute the 

 primary wall. No further development of this wall takes place until 

 the cell ceases to divide, except that the new middle lamella grows into 

 the surrounding walls and becomes continuous with their middle lamellae. 



When a cell has ceased to grow and divide it may develop a secondary 

 wall. This also consists of three layers, of which the middle one is much 

 the thickest. All three contain cellulose, but the thin outer and inner layers 

 contain more pectic substances than the middle layer. When lignification 

 of the wall occurs the lignin substances are laid down chiefly in the thick 

 cellulose layer, in the form of longitudinal radial plates, rich in lignin, 

 divided by layers rich in cellulose. Even in the walls of a woody cell there 

 is thus a large amount of cellulose, with which the lignin is mixed or perhaps 

 chemically combined. 



The deposition of the secondary wall layers is seldom, if ever, perfectly 

 uniform ; there are commonly a number of unthickened spots, which are 

 called pits, at which the cells are separated only by the primary wall. Through 

 this thin partition there frequently run extremely fine protoplasmic threads, 

 the plasmodesma or intercellular connections. Although the plasmodesma 



