euphycophyta 39 



* Chlorococcales 



This order contains a number of diverse forms, both in respect 

 of their morphology and also of their reproduction. Nearly all are 

 fresh-water or terrestrial. Some, because of their habit, e.g. Chloro- 

 chytrium, Chlorococcum, probably represent reduced forms. In 

 structure the plants range from soHtary, free-living or attached uni- 

 cells to large coenobia {Hydrodictyon) up to 20 cm. long. A charac- 

 teristic feature of the order is the lack of vegetative cell division, 

 though this has not precluded nuclear division, so that cells in some 

 species become multinucleate. In the various examples discussed 

 below it will be noted that motility has been suppressed more than 

 once in the class, a feature which, taken in conjunction v^th their 

 diverse morphology, suggests that they may well be polyphyletic 

 in origin. Some of the genera show indications of a relationship, 

 but there are others whose relationships are extremely vague. Some 

 forms {Trehouxia) have become associated with fungi and are an 

 integral part of lichens. They were originally classed with Chloro- 

 coccum (Cystococcus) (see below and p. 427). Sexual reproduction 

 when present is commonly by means of motile isogametes, but 

 oogarny has been recorded for a species of Dictyosphaerium. 



* Chlorococcaceae: Chlorococcum (chloro, green; coccum, berry). 

 Fig. 15 

 Much confusion has existed over this genus, and many of the 

 species formerly described are now knovm to be phases in the life 

 cycles of species from other genera. The plants are represented by 

 non-motile, spherical cells which vary much in size, occurring 

 singly or else forming a stratum on or in the soil. C. humicolum^ for 

 example, is a very common soil species. There is no eye spot or 

 contractile vacuole; the chloroplast is parietal with usually one 

 pyrenoid. The cell walls are two-layered with a thin inner layer and 

 an outer gelatinous one which is sometimes lamellose and irregu- 

 larly thickened. The young cells with thin walls are iminucleate 

 but the adult ones with thick walls are commonly multinucleate, 

 and it is in this older condition that the protoplast divides and 

 gives rise to 8, 16 or more biflagellate swarmers which are liberated 

 all together in a vesicle, usually in the early hours of the morning. 

 After a short motile phase the flagellae are withdrawn and a new 

 vegetative phase commences. 



