18 



PLANT MORPHOLOGY 



4. DINOPHYCEAE 



The Dinophyceae comprise a group of nearly 1,000 species of organisms, 

 most of which are known as dinofiagellates. Although some occur in 

 fresh water, most of them are free-swimming marine forms. A few have 

 naked protoplasts, but nearly all have sculptured walls of cellulose usually 



composed of a definite number of jointed 

 plates (Fig. 7). All the dinofiagellates are 

 unicellular and most of them are solitary ; 

 some occur in chain-like colonies. The 

 cells are small and generally have a pair 

 of laterally attached cilia (flagella). A 

 characteristic feature is the occurrence of 

 two grooves, one encircling the cell trans- 

 versely and the other extending longi- 

 tudinally along one side. The cilia arise 

 at the point of intersection of the grooves. 

 One lies in the transverse groove and the 

 other is directed backward. 



The dinofiagellates have a definite nu- 

 cleus and usually a number of brownish 

 yellow plastids, in which there is a predom- 

 inance of carotinoids over the chlorophyll. 

 Some are colorless. The colorless forms 

 live either as saprophytes or as animals, 

 the latter ingesting solid food particles. 

 Some are parasitic. Reserve food occurs 

 either as starch or as oil. Many of the 

 dinofiagellates are phosphorescent. The 

 prevailing method of reproduction is by 

 fission, but some members produce zoo- 

 spores. As in the other flagellates, cysts are often formed. Sexual repro- 

 duction has been reported in only one member of the class. 



In addition to the dinofiagellates, the Dinophyceae include a few forms 

 with a higher type of cellular organization, such as Gloeodinium, a palmel- 

 loid form, and Dinothrix and Dinodadium, both of which are filamentous. 



Fig. 7. Ceratium hirudinella, a 

 fresh-water dinoflagellate, X400. 



5. XANTHOPHYCEAE 



The Xanthophyceae,! or yellow-green algae, are a small but distinct 

 group of only about 200 species characterized by having an excess of 



1 Also called Heterocontae. 



