GROWTH 141 



chromosomes become evident, two poles or apparent centers 

 of attraction, arise in the cytoplasm on opposite sides of the 

 nucleus, and from each of these poles a spindle of delicate fibers 

 radiates inward toward the nucleus. The nuclear membrane 

 now disappears and the chromosomes become grouped in a plane 

 or plate stretching across the center of the cell at right angles to 

 the two series of spindle fibers, which soon meet and form on(» 

 continuous spindle reaching from pole to pole. At this point each 

 chromosome has become split lengthwise into two daughter 

 chromosomes, one of which now moves toward one pole and the 

 other toward the other pole. The whole chromatin mass is 

 thus divided exactly into two and the halves separate widely. 

 Each chromosome group now becomes broken up again into a 

 network around which a new membrane forms, and two complete 

 nuclei are thus produced. Meanwhile at the central point of 

 each spindle fiber appears a thickening, and these thickenings 

 soon enlarge and unite to form a disc or plate across the cell. 

 Along this plate a new cell-wall is laid down, which completes 

 the division of the mother cell into two similar daughter cells. 

 Why such a complicated process as mitosis should be necessary 

 in cell multiplication we do not understand, but it is perhaps 

 concerned with the need for making an exactly equal division of 

 the chromatin material, since this part of the nucleus is known 

 to be of great importance in directing the growth and differen- 

 tiation of the organism. 



Cell Enlargement. — Although two new cells have now been 

 formed, they still occupy together a space no larger than the 

 size of the original mother cell, so that no growth has as yet 

 really taken place. The abundance of sugar and other dissolved 

 foods, however, with which a growing region is always supplied, 

 causes a high osmotic concentration in these young cells, and 

 water is therefore vigorously drawn into them by osmosis. Since 

 the newly-formed walls are very thin and elastic they stretch 

 readily, and the small cells thus increase markedly in size until 

 their permanent bulk is attained. During this rapid expansion 

 the amount of protoplasm does not increase; and although it 

 fills the whole of the young cell it is necessarily restricted in these 

 larger ones to a thin sac which lines the wall. The bulk of the cell 

 is now occupied by the vacuole or sap-cavity so characteristic 

 of mature plant cells in general. This process of enlargement 

 by absorption of water, following the production of new cells by 



