218 



A TEXTBOOK OF BOTANY 



[Ch. V, 2 



of formation of all new cells by which the root increases in 

 length, the color being that of the abundant living proto- 

 plasm showing through the transparent walls. Third, 

 just behind the growing point lies a short smooth zone, which, 

 though little prominent, has yet this great importance, that 

 it is the growth zone, or place of enlargement to full size 

 of the new cells formed in the growing 

 point. The growth of the root in length is 

 wholly confined to this zone (though new 

 cells cause an increase in diameter farther 

 back), in great contrast to the conditions 

 in stems, where the growth occurs through 

 several expanding internodes (Figs. 112, 

 114). Fourth, just behind the growth zone 

 comes another, differing greatly in length in 

 different plants and under different condi- 

 tions, the root hair zone (Fig. 160). The 

 root hairs thereon show remarkably well, 

 especially through a lens, in roots germi- 

 nated from seeds in moist air, though they 

 have no such regularity of shape in the soil 

 (Figs. 161-2). In the former material the 

 hairs radiate very evenly outward, forming 

 collectively a sort of nimbus along the root ; 

 and they are obviously forming anew in 

 front, going each through its grand period, 

 Fig. 161. — Rad- and dying behind. Thus the zone moves 

 ish seedlings grown a i ong as a w h le just behind the advancing 



in moist air and m . . . , , 



soil. (From Sachs.) tip. The function of the hairs is well 

 known ; they provide the great surface 

 necessary for the absorption of the water when it is scant 

 in the soil. They pass this water through the cortex to 

 the ducts, which extend all the way from this zone to the 

 leaves. We can now see a reason why the entire growth 

 of the root in length takes place in advance of this zone, 

 for any growth behind the hairs would obviously tear them 



