44 DOUGLAS HOUGHTON CAMPBELL ON THE 



veloped, a sliort distance below the insertion of the leaf. Later ones arise from the broad 

 bases of the stipe, and such have their bundles connected Avith one of the leaf-bnndles. 

 The primary roots grow rapidly, soon breaking- through the overlying tissue; this taking- 

 place about the time that the tracheary tissue begins to form in the bundle with which 

 they are connected. 



The growth of the root from the apical cell is best studied in roots from an actively 

 growing plant, that is, in spring or early summer. Such roots are light colored at their 

 growing points, and their longitudinal sections through the tip will usually show very 

 satisfactorily the formation of the tissues. As seen from the side, the ajjical cell (PI. 

 7, fig. 6, a) is nearly triangular in outline, and in large roots is of very large size and 

 easily seen. In smaller ones, where growth is slow, it is smaller and not so readily de- 

 tected, although its form is often more regular, as in actively growing roots, the gi'owth of 

 the younger segments cut oft' from it often causes the sides to be bent (fig. 6). The outer 

 wall, which is usually broader than the lateral ones, is slightly convex. If a series of 

 transverse sections is made near the end of the root, one of them may pass through the 

 apical cell, which viewed from above appears also triangular (fig. 7), but the sides are 

 unequal. This shows that the cell, like the apical cell of the stem, is tetrahedral in form, 

 the base being directed toward the apex of the root. The apical cell contains a large 

 globular nucleus and the protoplasm is granular but not sufiiciently so to render the cell 

 opaque, in which respect less ditficulty is experienced than in the other parts of the 

 jjlant. 



The succession of the segments is very regular and can be traced Avith comjiaratively 

 little trouble. Each series consists of four cells, three from the lateral faces, which give 

 rise to the tissues of the root itself, and one from the outer face, the cap-cell, from which 

 the cells of the root-cap are formed. Each segment is at first a tabular cell, with the 

 broad faces triangular. In order to see the first di^-ision in each of the segments, trans- 

 verse sections through the young segments are necessary (PI. 7, fig. 7). If this is done 

 it will be found that each segment is first divided into two cells by a radial wall (sextant 

 wall). The sextant wall does not divide the segment into quite eqiuil parts, as its inner 

 edge is in contact with one of the lateral walls of the segment some distance above its 

 inner angle (fig. 8). Each of the two parts of the segment is next divided by a tan- 

 gential wall into two cells, an inner and an outer one (fig. 6). From the series of in- 

 ner cells the central fibi'O-vascidar bundle is derived; from the outer, the ground tissue 

 and cortex, each outer cell of the segment dividing- into two by a second tangential wall, 

 from the outer of which the cortex is formed, from the inner the ground-tissue. Almost 

 simultaneously Avith these divisions, others are formed parallel to the sextant Avails, but 

 for some time no walls are formed parallel to the Avail hy Avhich the segment Avas cut otf 

 from the apical cell, and consequently each segment remains in the form of a single layer 

 of cells, until about two full rows of segments have been cut off". As a result of the di- 

 AHsions of the segments being thus formed, the depth of the segments increases but little 

 as comjjared Avith the lateral groAvth, and the end of the root is nearly flat. 



The mass of meristem arising from the division of the segments, becomes A'ery soon 

 differentiated into three concentric masses. The central one of plerome is cylindrical 

 forming idtimately the fibro-vascular bundle, and, as before stated, arises from the inner 

 cells of the segments. Its cells become rapidly transformed, by longitudinal Avails, into 



