CEdogoniacew 57 



Order I. (EDOGONIALES. 



In this order the thallus consists of fixed, simple or branched 

 filaments. The cells possess a single nucleus and the chloroplast 

 is a parietal, more or less cylindrical, anastomosing mass of chloro- 

 phyll, containing one or more pyrenoids. The vegetative division, 

 in which a curious interpolation of new pieces of cell-wall takes 

 place, is peculiar to the order. The zoogonidia are also anomalous, 

 being characterized by a circlet of numerous cilia round the 

 anterior end. In the autumn, plants of this order frequently have 

 their cells packed with starch. The sexual organs are well-dif- 

 ferentiated oogonia and antheridia, and the sexual reproduction is 

 greatly specialized. There is only one family which includes three 

 genera, two of which are abundantly found in the British Islands. 



Family 1. CEDOGONIACE^. 



This family is represented in the British Isles by numerous 

 species of the two widely distributed genera (Edogonium and 

 Bnlbochcete. The young stages of these plants possess well-de- 

 veloped organs of attachment, but most of the species of (Edogo- 

 nium float freely in the water when adult. The thallus is simple 

 or branched and some of the cells exhibit a peculiar transverse 

 striation at their upper extremities. This is particularly notice- 

 able in the large species of (Edogonium and is the result of inter- 

 calary surface growth. Beneath one of the transverse cell-walls 

 an annular cushion of cellulose is deposited, and after each division 

 a circular split is formed in the cell-wall opposite this cushion, the 

 two parts remaining very slightly separated by a new piece of cell- 

 wall derived from the cushion of cellulose. The rings or cushions 

 of cellulose were investigated by Hirn 1 , who found that they 

 consisted of a central mucilaginous mass, surrounded by a coating 

 of cellulose formed as an inner cell-wall layer, which becomes 

 intimately concrescent with the old membrane above and below 

 the ring. After each division another slit is formed beneath and 

 close to the first one, the process being repeated until the upper 

 end of the cell frequently presents the appearance of having a 

 number of 'caps' placed one over the other (figs. 13 B and C; 

 14 A), each ' cap ' indicating a division of the mother-cell. 



1 Him in Acta Soc. Scient. Fennicae, xxvii, 1900. 



