20 INTRODUCTION. 



by side ; the arrangement of the inner cells, which may be called initial cells of 

 pleronie, and which suffer thenceforth divisions alternately in all directions, is therefore 

 often irregular. In the outer cells rapid divisions now follow, sometimes parallel to 

 the principal walls, sometimes radial, and tangential-perpendicular. The definite ar- 

 rangement of these is not ascertained. From these, only when they have attained 

 a very advanced stage of development, a superficial layer is marked off, which may 

 be called dermatogen. 



In the three fii"st stages of division of the segments (which only are well ascertained 

 in this case), the apex of the stem of Salvinia corresponds, according to Pringsheim's 

 statement, to that of Equisetum, with such differences as follow from the rows of seg- 

 ments being two in number. 



For the majority of the apices of stems of the Ferns ' it is doubtful, and requires 

 further investigation, whether and how far the first stages of division of the segments 

 correspond to the scheme derived from the simpler cases above described. At all 

 events, we know from the older statements of Hofmeister (Beitr. 11) that the segments 

 undergo directly many and repeated divisions, both in directions parallel to the prin- 

 cipal walls, and radial and tangential. By these the growing meristem is cut up into 

 many layers and rows of cells, which are arranged similarly to the segments, but in 

 which the boundaries of the single segments are not clear. A permanent layer of 

 dermatogen is first distinguishable after numerous tangential divisions ; a boundary 

 between plerome and periblem is for the present doubtful. 



The formation of the leaves begins, in plants which grow with an apical cell, 

 from an initial cell cut oflf from a segment ; and the leaf itself grows, at least in its 

 early stages, with an apical cell which forms segments (Fig. gA, d s). 



In the roots of the ferns in question, the formation of the cap starts also from 

 the apical cell, and begins by the cutting oft' of a cell from the otherwise unaltered 

 apical cell near its apical surface, by a wall perpendicular to the axis. This cell has 

 the form of a flat segment of a sphere, and is the first cap-cell (Fig. 7, 8, A, k). This 

 is the initial either of one of the simple layers of cells or sheaths (/, fu, n, p), from a 

 combination of which the root-cap is built up, or, by undergoing a subsequent 

 transverse division, it is the initial of a pair of such sheaths. Each primary cap-cell 

 is immediately divided by longitudinal walls at right angles to its outer surface, 

 which becomes more and more convex as the growth at the apex of the root 

 proceeds. It is divided first by a median longitudinal wall into two equal halves, 

 these being again divided into four quadrants by a wall at right angles to the first. 

 Each quadrant cell is again divided into two unequal parts by a longitudinal wall, 

 which halves the outer walls, or divides them into unequal parts, and then taking a 

 curved course inwards attaches itself to one of the lateral walls. The further divisions 

 which appear in the eight cells of the sheath thus formed become successively more 

 irregular, and may be followed up in the work of Nageli and Leitgeb. Where the 

 primary cap divides into two, this happens after the first three longitudinal divisions 

 are completed. 



According to Hofmeister, Hanstein, Nageli, and Leitgeb, the rule is that a 

 primary cap-cell is cut off from the apical cell after each cycle of segments, which go 



* On the phenomena in Ccratopteris, which differ from those in the other Ftr.s in the narrower 

 sense, compare Kny, Entwickl. d. Parkeriaccen, Abhandl. d. K. I.eop. Acad. Bd. 37(1875). 



