14 



GROWTH 



if ft 



SI 



lustrates the division of a plant cell. In its essential features it is 

 identical with the division of animal cells. Notice that the nu- 

 cleus fragments into a number of rods, the chromosomes, which 



by a regularly occurring series 

 of events are equally distrib- 

 uted to the two new, or daugh- 

 ter, cells where they are reor- 

 ganized into approximately 

 spherical nuclei. The result of 

 cell division is the formation 

 of two cells from one, each of 

 the new cells possessing a nu- 

 cleus in which there is half of 

 the substance of the chromo- 

 somes formed from the origi- 

 nal nucleus. This marvelous 

 and complicated process, re- 

 peating itself time after time 

 with wonderful precision, oc- 

 curs whenever a new cell is 

 formed and takes place in the 

 larger plants and animals mil- 

 lions of times during their 

 growth from a single cell to a 

 mature individual. An aver- 

 age-sized potato, such as you 

 might eat for dinner, has between five and six billions of cells. 

 Cell division must have occurred in its growth between five and 

 six billion times, each time without an error. The fundamental 

 cause of cell division is evidently an important problem from 

 the standpoint of growth because the increase in the size of or- 

 ganisms is due to an increase in the number of cells rather than 

 to increase in the size of the cells themselves. The cells in a 

 tomato plant are about the same size as those of a pine tree. The 

 cells in a pine tree one foot high are about as large as those in 



Figure g. Semi-diagrammatic representation 

 of the division of a plant cell. A. Resting 

 cell, the chromatin of the nucleus in a fine 

 network. B. The chromatin is gathered into 

 a long thread. C. This thread breaks up into 

 rods — the chromosomes. D. Each chomo- 

 some splits lengthwise. E. The nuclear mem- 

 brane disappears and the split chromosomes 

 are arranged in a plane across the equator 

 of the cell. F. The chromosome halves sepa- 

 rate, one complete set (eight in this case) 

 going to one end of the cell and the other 

 set to the other end. G. and H. Each group 

 of chromosomes is reorganized into a new 

 nucleus and a cell wall forms which results 

 in the formation of two new complete cells. 

 After Sinnott. By permission of McGraw- 

 Hill Book Company. 



