^^■^ 



VASCULAR CRFPTOGAMS. 



elevation of the older tissues (Fig. 261, £). The apex of the stem is always occupied 

 by a clearly distinguishable Apical Cell, which is either divided by walls alternately 

 inclined, and then resembles, when viewed from above, the transverse section of 

 a biconvex lens; or it is a three-sided pyramid, with a convex anterior surface 

 and three oblique lateral surfaces, which intersect behind. The oudines of the 

 segments, which are in the first case in two, in the second case in three rows, or 

 arranged with more complicated divergences, soon disappear in consequence of 

 numerous cell-divisions and of the displacement caused by the growth of the 

 masses of tissue and leaf-stalks surrounding the apex. The apical cell, for in- 

 stance, of Pieris aqiiilina, is wedge-shaped, the segments on the horizontal stem 

 forming a right and a left row ; the edges of the apical cell face upwards and 

 downwards (Fig. 260). The same is also the case, according to Hofmeister, in 

 Niphohohis dmiensis and rupestn's, Polypodium aureum and piinctulatum, and Plaiy- 

 ceriuni alcicorne. In Polypodium vulgare he states that it is sometimes •wedge-shaped, 

 sometimes pyramidal with three faces ; the last-named form occurs also in Aspidium 

 Filix-mas, Marattia cicutafolia, &c. As a rule it may for the present be assumed that 



Pig, 260.— Apical view of the end of the stem of Pteris aquiltna ; y the apical cell of the stem ; x the apical cell 

 of the youngest leaf ; h h hairs which cover the apical region surrounded by a cushion of tissue. 



creeping stems with a bilateral development have a wedge-shaped apical cell, upright 

 or ascending stems with radiating rosettes of leaves one that is a three-sided 

 pyramid. 



The further relationships of the segments of the apical cell of the stem to the 

 origin of the leaves and to the building up of the tissue of the stem itself, are still 

 but litde known in detail. It cannot be doubted that each leaf results from a 

 single segment only, and that this segment-cell is devoted from an early period to 

 the formation of the leaf, but it appears doubtful whether the segments always form 

 leaves, and if not what proportion of sterile segments precedes one from wdiich a 

 leaf is developed. 



The phyllotaxis of Ferns sometimes corresponds to the rectilinear arrangement 

 of the segments of the apical cell. Thus the distichous arrangement of the leaves 

 of Pieris aquilina, Niphoholus rupeslris, and of some species of Polypodium, corre- 

 sponds to the biseriate segmentation of the apical cell of the stem. But where the 

 phyllotaxis is complicated and spiral and the apical cell a three-sided pyramid, as 

 occurs in Aspidium Filix-mas^ the same processes may take place as in those Mosses 



