THALLOPHYTA: AWAE 



57 



Acetabularia. This is a marine genus occurring in tropical and sub- 

 tropical regions. It is called the mermaid's-wineglass. Acetabularia 

 crenulata is a common species off the coast of Florida and throughout the 

 West Indies. Its vegetative body, reaching a height of 6 to 9 cm., con- 

 sists of a stalk bearing rhizoid-like holdfasts below and expanded above 

 into a cup-like disk about 1 cm. in diameter (Fig. 45). The disk is com- 

 posed of a whorl of elongated branches that are laterally coherent, each 

 branch being a coenocyte. The plants are more or less incrusted with 

 lime. At first the plant has a single nucleus that soon gives rise to many 

 small nuclei. These pass up the stalk and 

 enter the disk, which has now become divided 

 into cells. 



Reproduction begins by the formation of 

 a large number of aplanospores (cysts) within 

 the fertile branches composing the disk. The 

 aplanospores are at first uninucleate but later 

 become multinucleate. They are liberated 

 into the water and, after a resting period, 

 each gives rise to a large number of biciliate 

 isogametes that escape and fuse in pairs. 

 The zygote germinates immediately to form a 

 new plant. The vegetative plant is diploid, 

 the reduction of chromosomes occurring when 

 the nucleus of the aplanospore divides. 



Acetabularia has been widely used by students of genetics and develop- 

 ment, especially in experiments on regeneration and polarity. 



Summary. The Siphonocladiales are multicellular algae with large 

 multinucleate cells, these usually containing many small chloroplasts. 

 The plant body is thus partially coenocytic. Vegetative growth takes 

 place by cell division. Asexual reproduction usually occurs by zoospores, 

 aplanospores, or akinetes. Sexual reproduction may be either isogamous 

 or heterogamous. This order is related both to the Chlorococcales and 

 to the Siphonales. 



Fig. 45. Acetabularia crenu- 

 lata, natural size. 



7. Siphonales 



The Siphonales are a distinct group of mostly marine algae, only a few 

 being found in fresh water. They are especially abundant in tropical 

 seas. As in the Siphonocladiales, many marine forms secrete lime. 

 Fossil members are known as far back as the Ordovician. The order 

 includes 50 genera and about 350 species. Representative genera are 

 Vaucheria, Codium, Bryopsis, and Caulerpa. 



Vaucheria. This well-known alga grows in felt-like masses in fresh 

 water and on damp soil. Some of its species live in brackish water and 



