48 GENERAL BOTANY 



cells with a large central sap cavity is either suspended in the 

 center of the cell by fine cytoplasmic fibrils, or strands, extend- 

 ing from. the nucleus to the cytoplasmic sac, or it lies embedded 

 in the cytoplasmic sac next to the cell wall. In any case a dense 

 envelope of cytoplasm surrounds the nucleus. 



The cell wall of all plant cells is formed by the living proto- 

 plasm, or protoplast, which finally occupies the cell cavity sur- 

 rounded by the secreted cell wall. In the development of a plant 

 from a fertilized egg cell the first wall is secreted by its proto- 

 plast as a protective envelope for the living protoplasm. All 

 subsequently formed walls are laid down between the two halves 

 of the protoplast in a dividing cell. The^ wall first formed is 

 aygjjiin and delicate and is composed of pectose. Upon this 



re is immediately deposited a whitish substance called cellu- 

 r ose, which is closely related to starch and sugar in its chemical 

 composition. Bleached celery is composed largely of the cellulose 

 which forms the cell walls, and a bleached celery stalk will give 

 the student a good conception of the general appearance and 

 nature of this cell-wall substance of plant cells. 



Although the first wall is always thin, it may become greatly 

 changed in its character as the cells of the plant body differ- 

 entiate to form the plant organs and tissues. These modifications 

 of the cell wall are brought about by two distinct methods: 

 first, by cell-wall thickening ; second, by changes in its chemical 

 character. 



The thickening of the cell wall may be effected by the de- 

 posit of new layers of cellulose on the primary thin wall by the 

 cell protoplast, in much the same way as a new layer of plaster 

 might be added to the wall of a house ; or the new wall substance 

 may filter into the cell wall and be deposited between the cellulose 

 particles of the original cell wall. The thick-walled cells which 

 form the supporting layers of stems and leafstalks (Fig. 26, b 

 and <?) are illustrations of thick-walled cells whose walls are 

 composed largely of cellulose, while the wood and bast fibers 

 (Figs. 52 and 53) from which wood pulp, linen, and hemp are 

 made are illustrations of thick-walled cells in which the chem- 

 ical character of the cell- wall substance has been changed by 



