EUPHYCOPHYTA I25 



fucosan, and they are particularly abundant in tissues where active 

 metaboHsm or division is taking place. 



Hyaline hairs occur in many forms and their function has been 

 variously ascribed as : 



(i) shock absorbers, 



(2) respiratory and absorptive organs, 



(3) protection against intense illumination, 



(4) protection against epiphytes, 



(5) protection against covering by sand or silt, 



(6) mucilage organs. 



None of the evidence for any of these suggestions is entirely 

 satisfactory, and the whole problem demands further investigation. 



Vegetative reproduction may take place by splitting of the thallus 

 or else by the development of special propagules (Sphacelarta). 

 Asexual reproduction is commonly secured by means of uni- or bi- 

 flagellate zoospores which are normally produced in specialized 

 cells or sporangia. In one group (Dictyotales) tetraspores replace 

 the zoospores, these bodies being produced in groups of four in 

 each sporangium on plants that do not normally bear sexual organs. 

 In yet another group (Tilopteridales) asexual reproduction is by 

 means of uni- to quadrinucleate monospores. The homologies of 

 these monospores have been subject to much speculation and they 

 have been variously regarded as equivalent to 



(a) ptopsigults of Sphacelarta, 



(b) simple forerunners of tetraspores, 



(c) degenerate tetraspores, 



(d) parthenogenetic ova. 



The second suggestion is perhaps the most satisfactory in our 

 present state of knowledge, especially when considered in relation 

 to the vegetative characters. Sexual reproduction ranges from iso- 

 gamy, with both gametes motile and characteristically bearing two 

 flagellae inserted laterally, through a series in which differentiation 

 first to anisogamy and finally to oogamy can be traced. 



The ova are not normally retained on the parent plant so that 

 fertilization takes place in the water, though the ovum may remain 

 attached by a long mucilaginous stalk (cf. p. 193). The change from 

 isogamy to anisogamy is also accompanied by a corresponding dif- 

 ferentiation of the gametangia. Electron microscope studies of the 

 antherozoids of several species (Manton, 1951, 1952) has revealed 

 the fact that the long anterior flagellimi is composed of eleven 



