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THE POPULAR SCIENCE MONTHLY. 



zone a short distance behind the apex of a rootlet. Later investi- 

 gators have confirmed Ohlert's conclusions, and have found that 

 the terminal organ, instead of being absorbent like a sponge, is in 

 reality a protective cap, and as impervious to water as cork. (See 

 Fig. 1.) 



Just behind this cap, and inclosed by it as a thimble covers 

 the finger-tip, lies that part of the root which is youngest and 

 tenderest, where growth is most vigorous, and from which all the 



Fig. 1. Parts of a Young Root (Pentstemon). (1) Seedling, with earth-particles attached 

 to the root-hairs. (2) The same, showing the root-hairs freed from earth -particles. 

 (3) Eoot-tip penetrating the soil ( x 10). (4) Root-hairs with earth-particles adhering 

 ( x 50). (5) Vertical section of root-tip, showing protective cap and growing point 

 (x 30). (Kerner.) 



other tissues of the root are derived. This vegetative point we 

 may consider as the tip proper. (See Fig. 1 (5).) As fast as the 

 surface wears off by contact with the earth, new tissue is added be- 

 neath, much the same as one's finger-nail is constantly renewed, 

 and thus the thickness of the cap remains about the same, al- 

 though continually worn away. 



The new tissue which is added to the body of the root soon 

 loses the power of increasing in length, and consequently the 

 elongation of a root is in marked contrast to the elongation of a 

 stem. The latter, to be sure, has, like the root, a small mass of 

 formative tissue at the apex, but the tissue which is formed con- 

 tinues to enlarge for a comparatively long time, and the result 

 is that a young stem grows in length at a nearly uniform rate 

 throughout, while in a rootlet elongation takes place only near 

 the tip. The simple experiment of making a series of equidis- 

 tant ink-dots along the stem and root of a bean seedling will, as 

 growth proceeds, give a good idea of the difference in manner of 

 growth. It is obvious that were a root to elongate like a stem, 



