390 



CEdogoniales 



AVille ('87) stated that his observations led him to the conclusion that only those young 



plants which had become attached were able to 

 grow and divide, the unattached unicellular 

 plants invariably forming zoogonidia again. 

 This is very often the case (fig. 245 A\ but 

 both Fritsch ('02) and West (G. S. W. '04) have 

 observed short free-floating young filaments 

 (vide fig. 244 G and /). In CE. fonlicola A. Br. 

 a basal disc is developed and in those plants 

 which become attached the margins of this 

 disc never become more than crenulate (fig. 

 245 C and D), whereas in unattached uni- 

 cellular plants the disc becomes much lobed 

 and branched (fig. 245 E H ; G. S. W., '12). 



It is not unusual for the young unicel- 

 lular plants to liberate their contents as a 

 zoogonidium after a day or two, and this 

 may be repeated through several generations 

 (vide fig. 245 A}. During this process the 

 apical cap either falls off or remains as a hinged 

 lid. After a time the young plants become 

 two-celled. Poulsen ('77) was the earliest 

 investigator to give a precise account of this 

 first division and Fritsch ( ; 02 B) largely con- 

 firms his observations. The cell-wall in the 

 apical region becomes thickened by the 

 development of an inner secondary membrane 

 the lower part of which forms a cellulose 

 ring at the upper extremity of the cell, very 

 like that formed during cell-division in the 

 adult filament, but whereas in the latter the 

 outline of the ring makes two acute angles 



with the inner limit of the cell-wall, in the unicellular plant there is only the lower acute 

 angle, the upper part of the ring going over into the inner layer of the wall above in a 

 gradual curve. A circular split is formed in the outer part of the ring, and by the growing 

 out of the contents of the old cell the ring is gradually stretched until it forms the lateral 

 wall of the new cell. The upper end of the old part of the cell-wall, which now forms the 

 first apical cap, is in some species thrown off, but in others it is permanently retained. 

 After the new lateral wall has been considerably stretched and elongated the first transverse 

 wall appears as a cell-plate. Subsequent divisions generally take place normally as in the 

 cells of the adult filament. 



In the second type of germination the basal cell swells out and becomes 

 either depressed-globose or somewhat hemispherical owing to a slight flattening 

 of the under-side by which it is attached (fig. 246). Attention was first 

 directed to this mode of development by Lemmermann ('98) and subsequently 

 both Scherffel ('01) and Fritsch ('04) have given a more extended account of 

 it. This basal cell has abundant chlorophyll and starch-grains. It also 

 possesses a small apical cap in the middle of the upper surface. On the first 



Fig. 244. Various stages in the develop- 

 ment of the zoogonidium in an unde- 

 termined species of (Edogonium. x 460. 

 This is the usual method of germination 

 with the formation of a basal rhizoid. 



