74 THE ELEMENTS OF VEGETABLE HISTOLOGY 



layers may be clearly marked or may be so consoli- 

 dated that but a single wall is apparent. Growth 

 by intussusception is of ten apparent in walls which show 

 thickenings at certain points and not at others. 

 Although the primitive cell membrane is composed 

 of cellulose, this substance is usually replaced by 

 other materials during the process of growth in the 

 wall. Lignified cell walls are those in which a woody 

 substance has been deposited upon the cellulose 

 membrane. Suberized cell walls are found in cork 

 cells, and result from the deposition of suberin upon 

 the cellulose. Cutinized walls occur in the covering 

 membranes of leaves where the original cellulose has 

 been replaced by cutin. 



ORIGIN OF CELLS AND TISSUES 



Every plant, unicellular or multicellular, is the 

 direct descendant of a preceding generation. In the 

 lower forms of plant life the production of new cells 

 is often a comparatively simple process and consists 

 in a direct division of the nucleus followed by the 

 formation of a wall between the divided nuclei. The 

 wall between the cells is formed by a constriction of 

 the original cell wall and an inward projection of the 

 latter, until the newly formed cell is separated from 

 the parent cell. In the higher forms of plant life 

 the process of cell division is extremely complex and 

 is termed mitosis or indirect nuclear division. Mitosis, 

 like the direct method of cell reproduction, consists 

 essentially in a division of the nucleus followed by the 

 formation of a cell wall between the separated nuclei, 

 but differs in the manner in which the nuclear division 



