104 



PART II. ANATOMY AND HISTOLOGY. 



29 



older, and persists only at the apex constituting the root-cap 

 (see p. 44). The only other important fact to be noticed at present 

 is that the root, as a rule, is monostelic. 



b. Growing-points iv ith a single apical cell are to be found in 

 stems and roots of most Pteridophyta : for instance, in the stems 

 and roots of all Leptosporangiate Filicinse, and in those of. 

 Ophioglossacese) ; in those of the Equisetinse ; and in those of 

 some species of Selaginella (S. Martensii and Kraussiana). The 

 shape of the apical cell is generally a three-sided pyramid with 

 a spherical base, the base being at the surface of the member 

 and the apex being directed inwards ; less commonly the apical 

 cell has only two sides or flanks and is then somewhat lenticular 

 in shape (e.g. that of the rhizome of Pteris aquilina, stem of 

 Salviniacese, and frequently in the above species of Selaginella). 

 In growing-points of this structure it is seen (Fig. 85) that the 



embryonic tissue- 

 systems are not 

 continuous as in 

 the Phanerogams, 

 but are inter- 

 rupted at the 

 apex by the large 

 apical cell. The 

 apical cell is, in 

 fact, the initial 

 cell for all the 

 three primary 

 tissue- systems. 



The apical cell undergoes division by walls formed parallel to 

 each of its (two or three) flanks successively, the segments thus 

 formed growing and dividing to form the tissues of the stem or 

 root. In the root the apical cell also undergoes divisions parallel 

 to its curved base. After the cutting off of a segment the apical 

 cell grows to its previous size, so that the repeated segmentation 

 does not diminish the bulk of the apical cell. 



The segments cut off parallel to the base of the apical cell of 

 the root (Fig. 8G k) constitute the derrnatogen. These dermatogen- 

 segments grow and divide both anticlinally and periclinally to 

 form the root-cap ; but this many-layered epidermis only persists 

 at the growing-point, since it becomes entirely exfoliated as the 

 parts grow older. 



FIG. 86. Diagrams illustrating the division of the apical 

 cell of the stem of Equisetum : A longitudinal section ; B sur- 

 face view. The numbers 1-7 indicate the successive segmental 

 walls; the fainter lines indicate the walls of subsequent divi- 

 sions of the segments. 



