54 



THE ALGAE 



MoNOSTROMACEAE : Monostroma (mono, single; stroma, layer). 

 Figs. 265 27 

 The thallus develops as a small sac, which in most species rup- 

 tures very early to give a plate of cells one layer in thickness, the 



ULOTHRIX MONOSTROMA 



Fig. 26 Diagram to illustrate the three different types of life 

 cycle found in the Ulotrichales. RD = place of reduction division 



in life cycle. 



cells often being arranged in groups of two or four. In M. grevilld 

 the thallus ruptures only in the adult stage, whilst in M. lindaueri 

 the sac frequently remains entire. Male and female gametes ( + 

 and - strains) are produced from separate plants and several 

 species exhibit anisogamy. The non-motile zygote increases in size 

 and after some months undergoes meiosis and commonly forms 

 thirty-two zoospores : in one species it has been shown that half of 

 these give rise to male plants and half to female plants. The macro- 



Fig. 27 Monostroma crepidinum. A, plant ( x f ). B, cells of thallus 



( X 200). C, transverse section of thallus ( x 200). D, M. lindauerii 



plants ( X I). (After Chapman.) 



