Simple Plants With Chlorophyll — Algae 157 



male and female sex cells. The fertilized cell is called a zygote and sur- 

 rounds itself with a thick, resistant wall to withstand adverse conditions. 

 Eventually, the single nucleus of the zygote produces four nuclei which 

 are incorporated into four zoospores, each of which forms a Ghlamydo- 

 monas plant. The zoospores and the gametes look alike except that the 

 former are larger. 



Protococcus (pro to -kok' us) (Gr. protos, first; kokkus, berry or 

 round), — This unicellular, thick-walled, round green alga is common on 

 trees and in moist places. Each cell has a nucleus and a lobed chloro- 

 plast. Reproduction occurs by cell division, and occasionally several in- 

 dividual cells may remain together to form a colony (Fig. 30) . 



Spirogyra (spi ro -ji' ra) (Gr. speira, coil or spiral; gyros, curved).- — 

 This green alga is common in fresh water where it may be called "pond 

 scum" or "water silk." Each unbranched filament is composed of a 

 linear series of cells and is covered with a slippery, mucilaginous sheath. 

 Each cell contains a single, organized nucleus located in the center of 

 the cell and surrounded by cytoplasm (Fig. 30). Strands of cytoplasm 

 also extend to the pyrenoids located on the chloroplasts. One or more 

 spiral-shaped chloroplasts may be present in a cell. 



Reproduction is by fragmentation (asexual) and by conjugation (sex- 

 ual). In the latter, the cells in two adjacent filaments form a conjuga- 

 tion tube between them, and the contents of one cell passes through the 

 tube to the cell of the other filament. This fusion, or fertilization, pro- 

 duces a zygote which eventually will produce a new filament. Even 

 though the gametes are all the same size (isogamous) , the one which 

 migrates might be considered as male and the other as the female. In 

 most cases, nearly all the cells of a certain filament produce gametes at 

 the same time. However, the cells of a single filament may unite at times 

 (Fig. 62). 



Ulothrix (u' lo thriks) (Gr. oulos, wooly; thrix, hair) . — This is a fila- 

 mentous, unbranched, fresh-water, multicellular green alga with a basal 

 holdfast cell for attachment to the substratum. The vegetative (body) 

 cells of the filament are differentiated and interdependent. Each vegeta- 

 tive cell contains an organized nucleus and a chloroplast which resembles 

 an open band or ring and which contains numerous pyrenoids (Fig. 30), 



Reproduction occurs by fragmentation, by zoospores, and by isogamy. 

 Certain reproductive structures are known as zoosporangia and each con- 

 tains two, four, eight, sixteen, or thirty-two large, motile zoospores. Each 

 zoospore bears four fiagella and forms a new filament by cell division. 



