54 HISTOLOGY OF MEDICINAL PLANTS 



chromatin — which are highly susceptible to organic stains. 

 Among the granules are thread-like particles or linin. Near 

 the centre of the nucelus are one or more small rounded nucleoli. 

 The liquid portion of the nucleus, filling the membranes and 

 surrounding the chromatin, linin, and nucleoli, is the nuclear 

 sap. 



Other cell contents characteristic of certain cells are crystals, 

 starch, aleurone, oil, and alkaloids. The detailed discussion of 

 these substances will be deferred until a later chapter. 



The cell wall which surrounds the protoplast is a product of 

 its activity. The structure and composition of the wall of any 

 given cell vary according to the ultimate function of the cell. 

 The walls may be thin or thick, porous or non-porous, and 

 colored or colorless. The composition of cell walls varies greatly. 

 The majority of cell walls are composed of cellulose, in other 

 cells of linin, in others of cutin, and in still others of suberin, etc. 

 In the majority of cells the walls are laid down in a series of 

 layers one over the other by apposition, similar to the manner 

 of building a pile of paper from separate sheets. The first layer 

 is deposited over the primary wall, formed during cell division; 

 to this is added another layer, etc. A modification of this 

 manner of growth is that in which the layers are built up one 

 over the other, but the building is gradually done by the deposit 

 of minute particles of cell-wall substance over the older de- 

 posits. Such walls are never striated, as is likely to be the case 

 in cell walls formed by the first method. In other cells the walls 

 are increased in thickness by the deposition of new wall material 

 in the older membrane. The cell walls will be discussed more 

 fully when the different tissues are studied in detail. 



INDIRECT CELL DIVISION (kARYOKINESIS) 



The purpose of cell division is to increase the number of cells 

 of a tissue, an organ, an organism, or to increase the number of 

 organisms, etc. Such cell divisions involve, first, an ecjual 

 division of the protoplast and, secondly, the formation of a wall 

 between the divided protoplasts. The first changes in structure 

 of a cell undergoing division occur in the nucleus. 



