OEDOGONIALES 57 



OEDOGONIALES 



^Oedogonium (oedo, swelling; gonium, vessel). Figs. 42, 43. 



The three genera, Oedogoniiim, Oedocladium and Bulbochaete, 

 which comprise this order were at one time classed as a separate 

 group, the Stephanokontae. Under the new scheme of classification, 

 however, they must be regarded, together with the other members 

 of the old Isokontae, as forming the Chlorophyceae. 



In Oedogonium the thallus consists of long unbranched threads 

 which are attached when young, though later they become free- 

 floating, whilst in the other two genera the filaments are commonly 

 branched. Each cell possesses a single nucleus together with an 

 elaborate reticulate chloroplast containing numerous pyrenoids. 

 The cell wall contains, according to some workers, an outer layer of 

 chitin, and if they are correct this is of great interest because chitin 

 is essentially an animal substance. The chromosomes of Oedogo- 

 nium are especially interesting among those of the algae in that they 

 have thickened dark segments at intervals along their length. 

 Vegetative cell division is so peculiar and characteristic that many 

 accounts of the process have appeared. A thickened transverse 

 ring, which develops near the upper end of the cell, first enlarges 

 and then invaginates, the much thickened wall being pushed into 

 the interior of the cell. Nuclear division now takes place near this 

 end of the cell and a septum is laid down between the two daughter 

 nuclei. Next, the outer parent cell wall breaks across at the ring and 

 the newly formed membrane stretches rapidly now that the 

 pressure is released — a matter of about 15 min. — so that a new cell 

 is interposed between the two old portions. The new transverse 

 septum becomes displaced by differential growth of the two 

 daughter cells so that it finally comes to rest just below the fractured 

 parent wall, and it is also evident that the new longitudinal wall of 

 the upper cell is almost entirely composed of the stretched mem- 

 branous ring. The old walls form a cap at one end and a bottom 

 sheath at the other, and as successive divisions always occur at the 

 upper end of the same cells, a number of caps develop there and 

 give the characteristic striated appearance to some of the cells. This 

 method of growth in Oedogonium may be either terminal or inter- 

 calary, but in the other two genera, as each cell can only divide 

 once, there is usually only a single cap. This peculiar mode of 



