GBOWTH AND CELL DIVISION 73 



network of the cell protoplasm in our figure. The nuclear wall, 

 or membrane, is a delicate bounding membrane which in the liv- 

 ing condition of the cell is composed of cytoplasm. Within this 

 membrane the nuclear protoplasm is seen to have essentially the 

 same structure as the cytoplasm, except that the bounding walls 

 of the meshes in the nuclear protoplasm are heavier and more 

 granular and stain more deeply in permanent preparations. This 

 deeply staining protoplasm of the nucleus is called chromatin 

 (from chroma, meaning " color ") on account of its avidity for 

 various stains used in preparing cells for microscopic study. 

 The chromatin substance is very essential in cell division and 

 in reproduction, and its distribution in the nucleus of the resting 

 cell is thus of extreme importance. In addition to the chromatin 

 network the figure shows a conspicuous nucleolus surrounded by 

 a characteristic nuclear vacuole. Under high powers of the micro- 

 scope the nucleolus appears to be suspended in this vacuole by 

 delicate strands of nuclear substance which connect the nucleolus 

 with the chromatin network, or spongework. With this prelimi- 

 nary review of the minute structure of a resting cell we may 

 proceed to consider the stages of cell division. 



CELL DIVISION MITOSIS 



Function. We have learned that the cells in the meristem .and 

 the elongating zones of growing root and stem tips increase in 

 number by cell division and that the cells thus produced grow 

 into the mature organs and tissue of the plant. The same thing 

 happens also when the single egg cell of the plant forms a many- 

 celled embryo by division. The embryo then differentiates and 

 grows into a new organism. The multiplying, or increase in 

 number, of cells in an organism for purposes of growth is there- 

 fore the most obvious use of cell division in plants. 



Although this production of new cells appears at first sight 

 to be the main object of mitosis, biologists in recent times have 

 called attention to the remarkable precision with which the chro- 

 matin substance of the nucleus is divided and to the complicated 

 mechanisms of mitosis by which this equal division is effected. 



