INTRODUCTION. 15 



The investigations of Bruchman assign to the root of Isbetes a conformity with 

 the third type of Angiospermous roots. According to the results of this observer, 

 which harmonize with those of Hegelmaier^ with the exception of. one not very imr 

 portant difference, the apeX of the stem of the Isoetese is occupied by a small group 

 of initial cells, which are common to the whole meristem. Longitudinal divisions of 

 these form the mother cells of the peripheral layers of meristem, and reiiew the 

 initial cells ; transverse divisions of them supply new elements to the central part 

 of the meristem. A division into the three distinct layers cannot be seen. 



Very similar to the structure of the apex of the stem of Isoetes is the 

 differentiation of the meristem peculiar to some Selaginellas, and the roots of Marattia. 

 It should therefore be mentioned here, but for the sake of shortness the description 

 of it will be supplied further on ^. 



IV. The stem of Isoetes and the above-named Selaginellas and Marattiaceae 

 form the transition between the forms of differentiation of the meristem already 

 described, and that which prevails for the great majority of the Pteridophyta (comp. 

 Fig. 7-9). The characteristic in these cases is this, that the entire meristem of the 

 apex originates from one single common initial cell, which is called, from its position 

 at the apex of stem and root, the apical celP. Successive bipartitions divide the 

 apical cell in each case into an apical part, which retains the original position and 

 form, this being compensated again by growth, and remains as the apical cell ; and 

 a basal inferior part, which is added to the growing meristem. The latter part is 

 termed the segment-cell*'. Further divisions of the segment-cell form the meristem 

 and later tissues. Each portion of meristem, which originates from a single segment- 

 cell, is called a segment. In roots, besides these processes, there is in addition the 

 formation of root-cap, which also originates from the apical cell ; this must now be 

 provisionally ignored. 



The apical cell (Fig. 7-9) has in most of the present cases the form of a three- 

 sided pyramid, with convex base, which is the apical surface (i.e. the outer wall) ; while 

 the sides are sunk in the meristem. This is the case in all roots of the plants in ques- 

 tion (except those of the Selaginellas, in which the form of the apical cell is doubtful), 

 and in the majority of the apices of stems. In other cases the apical cell has the form 

 of a two-edged wedge, the arched base and the point having otherwise the same ar- 

 rangement relative to the other tissues as in the cases with the three-sided cell: apex of 

 the stem of Salvinia, Azolla, many Selaginellas (S. Martensii^, Kraussiana), and Poly- 

 podiacese (Pteris aquilina), Polypodium rupestre. Lingua, aureum, punctatum, phyma- 

 todes, Platycerium alcicorne, stolons of Nephrolepis undulata according to Hofmeister". 



In the stolons of the last named species, as the apex becomes stronger, the 

 apical cell assumes the three-sided pyramidal form. In Polypodium vulgare it alter- 

 nates between the two (Hofmeister). 



In the seedling of Selaginella Martensii the apical cell of the main shoot, 



.'' Bot. Zeitg. 1874, p. 481. = [HoUe, K. Ges. d. Wiss. zu Gbtt. 8 Jan. 1876.] 



' ' Nageli, Zeitschr. f. wiss. Bot. II. p. 121 (1845), III. p. 157. 

 * Pringsheim, Jahrb. f. wiss. Bot. III. p. 491. 



' [M. Treub, Recherches sur les Organes de la Veg. dii Selaginella Martensii, Leide, 1877.] 

 ' Hofmeister, Beitr. zur Kenntniss der Gefasskryptogamen II. Abhandl. d. k. Sachs, Gesellsch. 

 d. Wissensch. Bd. V. 



