NATURE OF PLANTS 51 



water is drawn to them from the adjacent soil at a distance of 

 from four to six feet. 



23. Extent of the Root Surface. — It may seem surprising that 

 the roots are able to take up the large volumes of water that are 

 lost daily through transpiration. The spread of the root, how- 

 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 

 much greater depths are attained; for example, alfalfa 31 feet 

 and mesquite 60 feet. The most important increase in the ex- 

 tent 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 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, ^) and we now see that it has a very 

 complex structure, consisting of a protoplasmic mass surrounded 

 by a delicate membrane and traversed by a newtork of delicate 

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

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



