4 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 equal 
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. 
