56 ROOT GROWTH 



ever is much more extensive than we think, usually quite equal 

 to that of the branches, so that the absorbing portion of the root 

 is beneath the drip of the branches or exposed to the rains and 

 dews. Furthermore, when we come to add together the lengths 

 of the numerous rootlets, the extent of the total root system is 

 still more surprising. The total length of the roots of an oat 

 plant has been estimated at 154 feet; of a corn plant, at 1320 feet; 

 and of a squash plant at fifteen miles. These roots in ordinary 

 soil reach a depth of from three to five feet, but in dry regions 

 sometimes much greater depths are attained; for example, alfalfa 

 31 feet and mesquite 60 feet. The most important increase in 

 the extent of the root system is due to the root hairs, 480 having 

 been 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 ab- 

 sorbing surface of roots may be increased from five to ten times. 

 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 grow^th 

 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, ^) and w^e now see that it has a very 

 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 to a 

 considerable extent the activity of the cell and determines the 

 kind of cell that shall be formed and the work that it shall per- 

 form. As the nucleus enlarges the chromatin increases in amount 

 and often forms a ribbon-like structure (Fig. 31, B) which finally 

 contracts and divides, thus forming numerous small masses of 

 chromatin, called chromosomes (Fig. 31, C). At this stage in the 



