iv.] BO 7^ ANY. 15 



thousandths of an inch in diameter, and of which three 

 millions are estimated to be formed in twenty-four 

 hours. 



15. Cells which have ceased to divide gradually 

 grow into their permanent form, which in various cases 

 is very different. 



(a) Those of cork and pith do not alter very much 

 in shape, and finally, losing their protoplasm and cell- 

 contents which are absorbed into more active adjacent 

 cells, simply contain air. 



(b) Wood and bast-cells grow very much in length. 

 The protoplasm continues to secrete cellulose which 

 is added to the cell-wall and gradually makes it very 

 thick (see Par. 6). These, too, lose eventually their 

 living contents and contain only air or water. Other 

 cells may have their walls thickened in the same way 

 without becoming elongated. Vessels (Par. 9) are 

 formed by the partitions between rows of superposed 

 cells becoming absorbed. 



(c) In many cases the protoplasm, instead of 

 secreting thickening material which is added to the 

 cell-wall, forms various substances out of the fluids 

 which permeate through the cell-wall and mix with the 

 cell-sap. These remain imbedded in the protoplasm, 

 as in the case of starch grains, oily and fatty matters, 

 or grains of albuminoids ; or they are dissolved in the 

 cell-sap, as in the case of sugar and the substances 

 (alkaloids, &c.) which give so many plants useful or 

 noxious properties. 



(d) These substances often seem to fill the cell- 

 cavity to the exclusion of everything else, but the 

 remains of the protoplasm can generally still be traced, 

 though in a, very shrivelled state. 



3 



