4 8 



FORMATION OF NEW CELLS 



estimated on .01 in. of a root of corn 1/17 in. in diameter, and 

 230 on a square mm. of a pea root. 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 root surface. 



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

 the 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 



FIG. 31. Cell division in root of corn: A, cell with nucleus enlarging 

 preliminary to division I, linin; ch, chromatin. The central dark body is 

 the nucleus. B, later stage, the chromatin has increased and appears as a 

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

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

 spindle. I. D. Cardiff. 



complex structure, consisting of a protoplasmic mass surrounded 

 by a delicate membrane and traversed by a network of delicate 

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

 cleoli). The most important portions of the nucleus are the 

 minute particles (chromatin) which are associated with the linin 

 threads. It has been supposed that this substance controls the 



