178 EXTERNAL CONFORMATION OF PLANTS. 



alternating leaves arise with the divergence |^. When the segmentation of the apical 

 cell is into three rows, so that each new division-wall of the apical cell is parallel 

 to the last division-wall but two, as in Fontinalis, two rows of leaves result, 

 arranged spirally with the constant divergence ^. When the apical cell is a 

 three-sided pyramid, but the new walls which are formed in it are not parallel 

 to those already in existence, but oblique, so that all the segments, e. g. on the 

 anodic side are broader than those on the kathodic side, then the segments no 

 longer lie in three straight rows, but either three spirals or one only can be 

 recognised encircling the axis ; and since each ' segment' in this case {e. g. in Poly- 

 trichum, Catharinea, and Sphagnum^), developes into a leaf, the leaves are formed 

 in spiral arrangements, with divergences depending on the obliquity of the principal 

 walls of the segments to one another I These phenomena show clearly that when 

 each segment produces a leaf, the phyllotaxis depends on the manner in which the 

 new principal walls of the segments arise; and since the direction taken by the 

 segmentation of the apical cell depends again on causes of which we are at present 

 ignorant, the phyllotaxis must also finally be referred to these unknown causes. In 

 certain cases a reason may be given why, when the segmentation of the apical 

 cell is uniform, the positions at which the leaves are formed are nevertheless variable. 

 The segments of the apical cell in Fontinalis He, as in Equisetum, in three straight 

 rows ; but the solitary leaves stand in straight rows and spirally with the constant 

 divergence -J. In Equisetum, on the contrary, alternating whorls arise of leaves 

 which have grown together in the form of a sheath, because here, as Rees has 

 shown ^, the three segments of each cycle, arranged originally in a spiral manner 

 from want of uniformity in growth, are finally placed on the same zone, from which 

 a circular projection next grows out, on which the teeth of the sheath are formed. 

 From the want of uniformity in the growth of the segments, the causes of which 

 are at present unknown, still further differences, as compared with Fontinalis, are 

 introduced, in consequence of which the development of the whorls themselves 

 becomes alternate instead of superposed (as might be the case). If the processes 

 which take place in Marsilea (as Hanstein has described them *) are compared with 

 this, it is seen that the segmentation of the apical cell of the stem agrees in the 

 main with that of Fontinalis and Equisetum ; it is in three rows with a divergence -J. 

 As in Fontinalis, the leaves originate by a curving outwards of the segment-cells ; 

 but the leaves are in this case not arranged in three rows as in Fontinalis, nor in 

 whorls as in Equisetum, but in two rows. The immediate cause of this must be 

 sought in the fact that the stem, together with the piinctiim vegetationis, lies in a 

 horizontal position ; it has an upper and an under side The segments of the apical 

 cell form two rows on the upper and one on the under side ; but the former 



^ Compare the admirable description by Leitgeb in the case of Sphagnum, in Sitzungsber. der 

 kais. Akad. der Wissenschaften. Vienna, March 1869. 



^ Cf. Hofmeister, Allg. Morph. p. 494; and Miiller, Bot. Zeitg. 1869, a general morphological 

 study, t. IX. fig. 24. In such cases the behaviour of the apical cell can in fact be imagined as if it 

 rotated on its axis, as I supposed in my first edition. The description there given does not however 

 now appear to me suited to the beginner. 



^ Rees, Jahrb. fiir wissen. Bot. vol. VI. p. 216. 



* J. Hanstein, in Jahrb. fiir wissen. Bot. vol. IV. p. 252. 



