THALLOPHYTA: ALGAE 



41 



cross wall is developed across the parent cell, a method characteristic of 

 Ulothrix and multicellular algae in general. If a second wall appears in 

 one or both of the daughter cells before they separate, it comes in at right 

 angles to the first one. Later divisions may be in the third plane. Thus 

 there is a slight tendency in Protococcus toward the development of a 

 multicellular body. 



Protococcus is now generally regarded, not as a primitive form, but as 

 one that has become reduced from more highly developed ancestors, 

 probably as a result of its terrestrial mode of life. This is indicated by 



Fig. 29. Protococcus viridis, a unicellular green alga, X 1,000. Some of the cells have 

 divided to form small temporarj' groups. Each cell has a central nucleus and a peripheral 

 lobed chloroplast. 



its advanced method of cell division combined with a failure to develop an 

 extensive multicellular plant body like that of other Ulotrichales and by 

 the absence of zoospores and gametes, which even such truly primitive 

 forms as Chlamydomonas possess. 



Ulva. This is a widely distributed marine alga commonly known as 

 "sea lettuce." It grows along seacoasts between the high- and low-tide 

 lines. The vegetative body consists of a plate-like thallus two layers of 

 cells in thickness (Fig. 30). It is attached to rocks and other objects in 

 the water by means of a basal holdfast consisting of long colorless rhizoids. 

 The thallus may reach a length of 30 cm. or more. Each cell is uni- 

 nucleate and has a single chloroplast with a pyrenoid. 



Reproduction in Ulva closely resembles that of Ulothrix. Zoospores 

 arise from ordinary vegetative cells situated along the thallus margin, four 

 or eight zoospores being produced in each cell. They are liberated into 

 the water through an opening in the cell wall and swim by means of four 

 cilia. Upon germination, a zoospore gives rise to a plant that produces 

 only gametes. These are smaller than the zoospores, more numerous 

 (16 or 32 in a cell), and biciliate. Two similar gametes^ coming from 



' Although some species of Ulva are strictly isogamous, others produce two kinds of 

 gametes that differ slightly in si?e. 



