Page:The Oak.djvu/45

 If, in addition to these successive transverse sections, we examine a carefully prepared longitudinal section, cut so as to pass accurately through the median plane of the root, the comparison not only establishes the above conclusion, but it enables us to be certain of yet other facts (Fig. 6). Such a section shows the root-cap covering the tip as a thimble the end of the finger, and the rim of this root-cap is evidently fraying away behind; the cells of which it is composed die and slough off as the root pushes its way between the abrading particles of soil. Obviously this loss of worn-out tissue must be made good in some way, and closer examination shows how this occurs. The extreme tip of the root proper fits closely into the cap, and evidently adds cells to the inside of the latter, and thus replaces the old ones which are worn away. At this true tip of the root, moreover, we make another discovery, namely, that all the cells are there alike in shape, size, and other peculiarities; and if we could take a