252 PART III. THE CLASSIFICATION OF PLANTS. 



nodes and internodes in regular succession ; from the nodal cells of the leaf 

 arise whorls of leaf-branches or leaflets. The only fundamental difference be- 

 tween the leaf and the stem of the Characese is that, whereas the apical growth 

 of the latter is unlimited, that of the former is limited ; the apical cell of the 

 leaf at length ceases to divide, assuming a somewhat cylindrical form with a 

 pointed tip. 



The roots, with the exception of the first root of the embryo, are all adventi- 

 tious, being developed from the lower nodal cells of the stem. They are 

 simpler in structure than the stem or leaf, each consisting of a colourless fila- 

 ment of long, narrow cells; the growth is apical, though the apical cell is not 

 specially differentiated as in the stem ; the cells of the root are connected in a 

 peculiar manner, the contiguous ends of the two cells having each somewhat 

 the shape of the sole of a human foot ; root-branches are developed from that 

 portion of the cell, just above the articulation, which corresponds to the heel of 

 the foot. 



The sexual organs (Fig. 181) are in all cases borne on the leaves : the an- 

 theridium is developed from the terminal cell of a leaf or of a leaflet; the 

 oogonium replaces a leaflet. The plant may be either monoecious or dioecious. 



The antheridium is a spherical body, of a green colour when young, but 

 orange when mature, borne on a stalk. Its wall consists of eight cells, each of 

 which is termed a shield, presenting marginal infoldiugs of the wall ; the wall of 

 the upper half of the antheridium consists of four triangular shields ; that of 

 the lower half consists likewise of four triangular shields, each of which has 

 its lower angle truncated to admit of the passage of the stalk-cell. On the 

 inner surface of each shield, at its centre, is attached a cylindrical cell, the 

 manubrium, which extends to near the centre of the antheridium. Each manu- 

 brium bears at its inner end a somewhat spherical cell, the capitulum. To 

 each capitulum are attached usually six rounded cells, the secondary capitula. 

 Connected with each secondary capitulum are two cells each of which bears a 

 pair of long filaments, each filament consisting of about two hundred cells. 

 The cells of the filaments are the mother-cells of the spermatozoids, each cell 

 giving rise to a single spermatozoid. 



The male cell or spermatozoid consists of a club-shaped spirally- wound mass 

 of protoplasm bearing two long cilia at its pointed anterior end (see p. 116). 

 When the antheridium is mature the shields separate, the spermatozoids are 

 set free from their mother-cells and escape into the water. 



The oogonium is the enlarged terminal cell of the leaflet which it represents. 

 Beneath the oogonium proper is a node, the central cell of which constitutes 

 the stalk-cell of the oogonium, whilst the five peripheral cells of the node grow 

 out into filaments which gradually become spirally twisted and enclose the 

 oogonium ; the tips of these filaments project at the free end of the oogonium, 

 constituting the croion or corona, and are cut off from the rest of the filaments 

 either by one transverse wall, so that the crown consists of five cells as in the 

 Chareae, or by two transverse walls, so that the crown consists of ten cells as in 

 the Nitelleae. 



Each oogonium contains a single oosphere, a nucleated mass of protoplasm 

 containing starch-granules, with a well-marked clear area, the receptive spot, at 

 the apical end. Just previously to fertilisation one or more cells (wendungs- 



