204 STRUCTURAL BOTANY 



are separated, the smaller of which grows out and 

 becomes the apical cell of the new branch. When a 

 branch terminates in a long colourless hair, its growth is 

 at an end ; these branches therefore are of limited 

 length, and in this respect resemble leaves, while other 

 branches retain their apical cell, and are thus capable of 

 indefinite growth. Hence the thallus of a Callithamnion 

 comes to have a regular conformation like that of many 

 higher plants, depending on the relative position of its 

 unlimited and limited branches. 



2. KEPRODUCTION 

 a. Asexual 



The reproduction of Callithamnion and of most FlorideaB 

 is of two kinds, asexual and sexual. The asexual 

 reproductive cells are called tetraspores, because they are 

 almost always produced four together in one sporangium. 

 In this case the tetrasporangia occur on very short 

 lateral branches (see Fig. 88, A), the end cell of which 

 swells up and becomes filled with exceptionally abundant 

 protoplasm and plastids, assuming a very dense red 

 colour. The contents then divide up into four spores, 

 arranged in this particular plant in a tetrahedron (see 

 Fig. 88, A). 



The membrane of the sporangium ruptures, and the 

 tetraspores are set free ; when they escape they are 

 without any cell-wall, each spore containing a single 

 nucleus. These spores have no cilia, and usually appear 

 to be quite incapable of any spontaneous movements ; 

 no doubt they are disseminated by currents in the water. 

 When a tetraspore comes to rest it forms a cell-wall and 



