VEGETATIVE OKGANS OF PLANTS. 



265 



To ascertain the total length of root, Hellriegel weighed 

 and ascertained the length of selected average portions. 

 "Weighing then the entire root-system, he calculated the 

 entire length. He estimated the length of the roots of a 

 vigorous barley plant at 128 feet, that of an oat plant at 

 150 feet.* He found that a small bulk of good fine oi' 

 sufficed for this development ; ^ 5 cubic foot (4 -{- 4 -j- 2; 

 in.) answered for a barley plant, ^ cubic foot for an 

 oat plant, in these experiments. 



Hellriegel observed also that the quality of the soil in- 

 fluenced the development. In rich, porous, garden-soil, 

 a barley plant produced 128 feet of 

 roots, but in a coarse-grained, com- 

 pacter soil, a similar plant had but 80 

 feet of roots. 



Root Hairs. The real absorbent 

 surface of roots is, in most cases, not 

 to be appreciated without microscopic 

 aid. The roots of the onion and of 

 many other bulbs, i. e. , the fibers Avhich 

 issue from the base of the bulbs, are per- 

 fectly smooth and unbranched through- 

 out their entire length. Other agricul- 

 tural plants have roots which are not 

 only visibly branched, but whose finest 

 fibers are more or less thickly covered 

 with minute hairs, scarcely perceptible 

 to the unassisted eye. These root-hairs 

 consist always of tubular elongations of 

 the external root-cells, and through 

 them the actual root-surface exposed 

 to the soil becomes something almost 

 Fig. 38. incalculable. The accompanying fig- 



ures illustrate the appearance of root-hairs. 

 Fig. 38 represents a young mustard seedling. A is 



Rhenish, 34 = 46 Bnglish feet. 



