1 66 The Living Plant 



one centers observation more exactly on these hairs, he will see 

 that nearest the tip they are plainly just forming, while farther 

 back they are progressively longer, until a maximum is reached, 

 behind which they are obviously withering and dying. Evi- 

 dently the single hairs have each their little day 

 and pass, while the zone as a whole moves forward 

 in perpetual youth, pari passu with the advancing 

 tip of the root. These hairs are of first importance 

 to our immediate subject, for they are the active 

 water-absorbing parts of the roots. 



Thus much can be seen with the eye and a 



lens, but hardly anj'thing more. If, however, one 



cuts a thin section through the apex and along the 



central axis of a root, and magnifies this section 



, with the microscope, he will have before him an 



Fig. 52.— a seed- , ^ ' 



ling of mustard arrangement like that of our picture (figure 53), 

 saturated air; with whicli it will be dcsirable to compare the 

 natural size. generaUzed section of figure 139 C. At the tip is 

 the root-cap, a cluster of cells which, continually renewed from 

 behind, acts as a protection to the delicate tip in its passage 

 through the rough and abrading soil; just behind lies a prominent 

 focal center, the growing point, whose closely-packed cells are so 

 densely filled with protoplasm that the characteristic yellowish 

 color of that substance shows through to the outside; while radi- 

 ating back from the growing point run long lines of cells which 

 gradually merge into the differentiated tissues of the older root. 

 These latter tissues, so far as they concern our immediate subject 

 of absorption, are shown generalized in figure 54, D. Of the lines 

 of cells, a few in the center constitute the pith, outside of which lie 

 the long lines of water-tubes, or ducts, readily identified by their 

 distinctive spiral markings. These ducts contain water, but, 

 contrary to what one would expect, are otherwise empty tubes, 

 possessing no living protoplasm after once they are formed; and 

 they run in continuous strands from the tips of the roots all 



