dp:velopment of the ostrich fern. 45 



a mass of iiai-row cells, the procambium (PI. 7, fig. 6, z). Lying immediately aromid 

 the plerome cylinder is a layer several cells in thickness, whose cells are liroader than 

 those of the plerome and which can be traced back to the inner of the two cells, 

 formed by the second division of the semi-segments. Tliis is the periblem (fig. 6, y) 

 giving rise to a ground tissue. Outside is the third layer, or dermatogen, arising from 

 the outer cells of the segments and forming the epidermis and cortical parenchyma, or 

 such hyjioderma as may be present. The limits of these three primary tissue-masses can 

 be traced l)ack to the youngest segments, as thei-e is little disjjlacement of the original 

 cell walls through subsequent growth, this being also true in cross-sections (fig. 8), the 

 sextant walls being perceptible in sections made some distance from the apex, and in such 

 sections the successive divisions of the plerome cells can best be studied. Each of the 

 original jjlerome cells is first divided by a tangential wall into an inner and an outer cell, 

 the latter being again quickly divided by a radial wall. The inner cell seems to have the 

 same process repeated again, but beyond this the divisions do not appear to follow any 

 regular order. In the periblem, no definite oi-der could be detected in regard to the for- 

 mation of the division walls, but in the dermatogen cells of the seventh or eighth seg- 

 ment from the apical cell walls are formed almost simultaneousl3% at right angles to the 

 longer axis of the cells, so that the segment, seen from the surface, shows a double row 

 of cells of nearly equal size. As a result of this division, the cells of the dermatogen 

 ai'e shorter than those of either plerome or periblem. 



All of the meristem cells have large nuclei and granular, but almost perfectly color- 

 less, contents, and usually sufficiently transparent to make their study an easy matter. 



The first division in the cap-cell is nearly parallel to one of its lateral walls, and the 

 cell seen from above appears divided into two cells, one of which is triangular, the other 

 quadrilateral. The segment soon loses its triangular form, becoming nearly circular and 

 a second wall is formed perpendicidar to the first, thus dividing the segment into four 

 nearly equal cells, arranged like the quadrants of a circle. These subsequently become 

 further divided by both radial and tangential walls, but not by walls parallel to the outer 

 wall of the apical cell, so that they retain permanently the condition of single layers of 

 cells. 



The wall ])y which the cap-cell is first separated from the apical cell is not perfectly 

 parallel to the outer Vail, but is somewhat flatter, so that seen at first the segments are 

 somewhat higher in the middle than at the sides. As it develops, the cells in the center 

 elongate more rapidly than those at the sides so that this inequality becomes more and 

 more marked as the segments become oldei', and the tip of the root finally becomes de- 

 cidedly conical, althongh the end of the root itself is nearly' flat. 



In the rapidly growing root, only the outer cells of the root-cap are brown, l)ut in the 

 more slowly -growing ones, all l)ut those near the apical cell have their walls somewhat 

 thickened and brown, and the cell-contents dried up. 



The diflerentiation of the parts of the fibro-vascular bundle from the plerome does not 

 begin until that of the ground tissue, hypoderma and epidermis is far advanced. At this 

 time, the plerome consists of a mass of narrow procambium cells, those toward the cir- 

 cmnference being somewhat shorter and with less oblique ends ; those of the central ones 

 being generally strongly oblique, so that the cells derived from them have sharply pointed 

 ends, fitting closely together with no intercellular spaces. The cells increase greatly 



