SECT. 1 



CRYPTOGAMS 



363 



a number of swarming spermatozoids. The spermatozoids are very small, and have 

 a single nucleus and two cilia inserted on one side. They collect around the 

 receptive spot of the egg-cell, into which one spermatozoid finally penetrates. 

 After the egg-cell has been fertilised by the fusion of its nucleus with that of 

 the spermatozoid, it becomes invested with a wall and converted into a resting 

 oospore. On germination the oospore grows into a filamentous thallus. 



CLASS X 



Characeae (Stoneworts) ( 6>36 ) 



The Characeae form a sharply defined group of Thallophytes, 

 the origin of which must be looked for in the Chlorophyceae, from 

 which, however, they are distinguished 

 by the complicated structure of their 

 sexual organs. They grow in fresh or 

 brackish water, 'attached to the bottom 

 and covering extended areas with a mass 

 of vegetation. In some species their 

 cylindrical main axes are over a foot in 

 length, and are composed of long inter- 

 nodes alternating with short nodes, from 

 which short, cylindrical branches are given 

 oft' in regular whorls with a similar 

 structure, but of limited growth (Fig. 

 298). The lateral axes are either un- 

 branched or give rise at their nodes to 

 verticillate outgrowths of a second order. 

 From the axil of one of the side branches 

 of each whorl a lateral axis resembling 

 the main axis is produced. The attach- 

 ment to the substratum is effected by 

 means of branched rhizoids -springing 

 from the nodes at the base of the axes. 



Both the main and lateral axes grow 

 in length by means of an apical cell, from 

 which other cells are successively cut oft' 

 by the formation of transverse walls. 

 Each of these cells is again divided by a 

 transverse wall into two cells, from the 

 lower of which a long, internodal cell 

 develops without further division ; while 

 the upper, by continued division, gives 

 rise to a disc of nodal cells, the lateral axes, and also, in the lower 

 portion of the main axis, to the rhizoids. In the genus Nitella the 

 long internodes remain naked, but in the genus Chara they become 



Flo. 298. Chara fragttis. End of 

 main shoot. (Nat. size.) 



