THALLOPHYTA: ALGAE 45 



4, Oedogoniales 



The Oedogoniales are related to the Ulotrichales and are often classified 

 with them. They are a fresh-water group including onl}^ 3 genera and 

 approximately 400 species. The two chief genera are Oedogonium and 

 Bulbochaete, both occurring throughout the world. 



Oedogonium. This widely distributed alga, comprising nearly 300 

 species, generally lives in ponds, lakes, and quiet streams, often attached 

 to sticks, stones, and other aquatic plants. It consists of a simple un- 

 branched filament that, when young, has a basal holdfast cell but later is 

 usually free-floating. The cells are elongated and uninucleate. Each 

 contains a peripheral chloroplast with many pyrenoids. The chloroplast 

 is band-like and reticulate. Any vegetative cell except the basal one 

 may divide. 



Oedogonium has a peculiar method of cell division seen only in the other 

 members of its order (Fig. 33). It results in the formation of distinctive 

 "apical caps." The nucleus divides near the upper end of the cell, where 

 simultaneously a ring-like thickening of cellulose is developed on the 

 inside of the lateral wall above the dividing nucleus. A groove appears in 

 this ring and the cell wall splits transversely opposite the groove. A thin 

 cross wall now appears between the daughter nuclei and the protoplast is 

 divided in half. The ring stretches into a cylinder as each daughter 

 protoplast elongates, the new cross wall moving upward to the top of the 

 parent cell, where it unites with the lateral wall very close to where the 

 transverse split occurred. The upper cell, which has a new cell wall, con- 

 tinues to elongate until it reaches the size of the lower cell, which possesses 

 the old cell wall. 



Asexual reproduction occurs by the formation of large zoospores, each 

 of which arises from the entire contents of an ordinary vegetative cell 

 (Fig. 34: A, B). This escapes as a naked protoplast that bears a crown of 

 cilia. The liberation of the zoospore is accompanied by a transverse 

 splitting of the cell wall at the apical end. After a period of free swim- 

 ming, the zoospore comes to rest with its ciliated end downward, retracts 

 its cilia, forms a cell wall, and gives rise by repeated divisions to a new 

 filament. Oedogonium may also produce akinetes, although these are 

 relatively uncommon. The akinetes may occur either singly or in a 

 linear series. They germinate directly into new filaments. 



Oedogonium is heterogamous. An antheridium arises as a short cell 

 that is cut off at the apex of an ordinary vegetative cell. It may remain 

 the only one, but generally more (from 2 to 40) are produced by continued 

 division of the lower cell or by division of antheridia already formed (Fig. 

 34C). Each antheridium gives rise to one or, more commonly, to two 

 sperms, either by a vertical or a transverse division of the protoplast, 



