Page:Popular Science Monthly Volume 38.djvu/42

32 zone a short distance behind the apex of a rootlet. Later investigators 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

 (Pentstemon). (1) Seedling, with earth-particles attached to the root-hairs. (2) The same, showing the root-hairs freed from earth-particles. (3) Root-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 beneath, much the same as one's finger-nail is constantly renewed, and thus the thickness of the cap remains about the same, although 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 continues 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 equidistant 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,