VACUOLATION AND GROWTH OF CELLS IO7 



state and assume the characters of permanent tissue 

 cells. The power of cell division does not necessarily 

 cease, but divisions become less frequent and in some 

 cases do not recur. 



The permanent tissue cells commonly attain a size 

 far greater than that of the meristematic cells — many 

 of them grow especially in the direction of the axis 

 of the root or stem, and become several times longer 

 than they are broad. This great increase in the size 

 of the cell does not result, however, from increase in 

 hulk of the protoplasm, but from the formation and increase 

 in size of vacuoles. Drops of liquid (cell sap) appear 

 in the endoplasm, increase in size (Fig. 6, B), and 

 eventually run together, forming one large vacuole, 

 in the midst of which the nucleus, always surrounded 

 by a layer of cjrtoplasm, is suspended by strands of 

 cytoplasm {bridles), which run across the vacuole to 

 the layer lining the cell wall. When the vacuole 

 increases still further in size the bridles are thinned 

 out till they collapse, and the whole of the cytoplasm 

 then forms a layer on the cell wall, the centre of the 

 cell being completely occupied by the large vacuole 

 (Fig. 6, C, left-hand bottom cell). 



During the process just described the appearance 

 and increase in size of the vacuoles distends the ex- 

 tensible cell wall, which would be thinned out to the 

 point of rupture were it not that new cellulose is 

 continually added to it by the protoplasm lining its 

 inner surface. When the cell has reached its definitive 

 size this process of addition of cellulose often con- 

 tinues, and now the cell wall is actually thickened, 

 the successive layers added to its inner surface being 

 often clearly visible on each side of the original thin 

 wall {middle lamella). This thickening is not, however. 



