OEDOGONIALES 6i 



thallic and they form the nannandrous group. The dwarf males 

 arise from motile androspores which are formed singly in flat 

 discoid cells, the androsporangta, produced by repeated divisions of 

 ordinary vegetative cells. The androspores may be formed either 

 in the oogonial filament — gynandrosporous species — or on other 

 filaments that do not bear oogonia — idioandrosporous species (fig. 

 43). In shape and structure the androspores are small editions of 

 the zoospores, and after swimming about they settle on the wall of 

 the oogonium or on an adjacent cell and germinate into a small male 

 plant which is composed of a rhizoidal holdfast with one or two flat 

 antheridia above, though in some cases only one antheridial cell 

 without any rhizoidal portion is formed. Usually two antherozoids 

 are freed from each antheridium into a delicate vesicle which later 

 dissolves. The antherozoids are also hke small zoospores, and if 

 they fail to enter an ovum immediately they may remain motile for as 

 long as 13 hours. In the macrandrous monoecious species the 

 antheridia are usually to be found immediately below the oogonia 

 where they arise by an ordinary vegetative division in which the 

 upper cell subsequently continues to divide rapidly, thus producing 

 a series of from two to forty antheridia. The antheridia frequently 

 develop one day later than the oogonia, thus ensuring cross- 

 fertilization. 



The oogonia are enlarged spherical or ellipsoidal cells arising by 

 one division in which the upper segment forms the oogonium and 

 the lower a support cell, or else the latter subsequently divides to 

 give antheridia. In some species the lower cell may also become an 

 oogonium so that one can find a series of oogonia on one filament. 

 Each oogonium contains one ovum with a colourless receptive 

 spot situated opposite to the opening in the oogonium wall from 

 which a small quantity of mucilage is extruded. The opening is 

 either a very small pore, formed by gelatinization of a tiny papilla, 

 or else a slit, but in either case there is an internal membrane 

 forming a sort of conduit to the ovum. After fertilization the 

 oospore often becomes reddish in colour and develops a thick 

 membrane which is usually composed of three layers. At germina- 

 tion the protoplast divides into four segments, which may each 

 develop flagellae and escape as zoospores, or else they function as 

 aplanospores that later give rise to zoospores. Meiosis takes place 

 at the germination of the zygote so that the adult filaments are 



