4» 



l-< IRMATK >\ ( >F NEW CELLS 



estimated on .oi in. of a root of corn i 17 in. in diameter, and 

 230 on a square mm. of a pea r<x»t. By this means the absorbing 

 surface of roots is increased from fifteen to seventy-five times. 

 It should be stated that these measurements have been obtained 

 under unsatisfactory conditions and therefore only approximately 

 represent the extent of the n><>t surfa 



24. The Structure of the Root. — You have noticed that only 

 tlu- apical portion of the root elongates and it may be well to first 



examine the structure of this region in order to see how growth 



is brought about and what changes are effected by it. The tip 

 of the root is composed of very delicate cells. Certain of these 

 cells are rapidly dividing by forming new walls through the 



middle of the cells, thus dividing each cell into two new cells. 



The division of a cell is effected in a very elaborate manner. The 

 first indication of this growth appears in the nucleus. This organ 

 of the cell enlarges ( Fig. 31, A) and we now see that it has a very 



ch " 



Fig. 31. Cell division in root of corn: ./, cell with nucleus enli 

 preliminary to division—/, linin; ch, chromatin. The central <l;irk body ia 

 the nucleus. />'. later stage, the chromatin has increased and appears 

 ribbon-like k- in. C, formation of the chromosomes, cr. D, formation of 

 the spindle and the arrangement of the chromosomes in the center of the 

 s|)in<lli I. I >. < Cardiff. 



complex structure, consisting of a protoplasmic mass surrounded 

 by a delicate membrane and traversed b\ a network of delicate 

 threads (linin) and containing one or more large granules (nu- 

 cleoli). The most importanl portions of the nucleus arc the 

 minute particles (chromatin) which are associated with the linin 

 threads. It has been supposed that this substance controls the 



