EUPHYCOPHYTA 67 



algae in that they have thickened segments at intervals along their 

 length. 



Vegetative cell division is so pecuHar and characteristic that 

 many accounts of the process have appeared. A thickened trans- 

 verse 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 fifteen minutes — 

 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 evident that the new longitudinal 

 wall of the upper cell is almost entirely composed of the stretched 

 membranous 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 same place, a number of caps develop there and give a 

 characteristic striated appearance to some of the cells. This method 

 of growth in Oedogonium may be either terminal or intercalary, but 

 in the other two genera, as each cell can only divide once, there is 

 usually only a single cap. This peculiar mode of division is unique, 

 and although there is no trace of its ancestry, its constancy suggests 

 that the group terminates a line of evolutionary development. 



Vegetative reproduction commonly occurs by means of frag- 

 mentation, whilst asexual reproduction is secured through akinetes 

 or multiflagellate zoospores, the formation of the latter being said 

 to depend on the presence of free carbon dioxide in the water. The 

 flagella, which may have one or two rings of granular blepharo- 

 plasts at their base, form a circular ring around an anteriorly situ- 

 ated beak-like structure. This is the typical oedogonian swarmer, 

 one of which is produced by each cell, and there are two theories 

 that have been put forward to explain its origin : 



(a) The group arose independently from flagellate organisms 



which possessed a ring of flagellae. If this is true then there 



could be no real connexion with the other members of the 



Chlorophyceae. 

 (h) Several divisions of the two original blepharoplasts and 



flagella took place, thus resulting in the ring structure. If 



