THALLOPHYTA: ALGAE 



51 



plast has two haploid nuclei. Then one nucleus in each protoplast 

 degenerates. 



Mougeotia. This alga consists of a deli(;ate unbranched filament. 

 Each cell displays a nucleus and a peculiar, axial, plate-like chloroplast 

 containing two or more pyrenoids (Fig. 38/1, B). The chloroplast can 

 change its position in the cell, presenting its flat surface to dull light and 



BCD 



Fig. 39. Spirogyra. A, a vegetative cell, showing the central nucleus and the peripheral, 

 band-like, spiral chloroplast with many pyrenoids, X500; B, C, D, stages in conjugation, 

 X250. 



its edge to bright light. Reproduction occurs by fragmentation, aplano- 

 spores, and by the conjugation of isogametes. The cells of two filaments 

 lying parallel to each other put out short bud-like outgrowths that come 

 into contact and form tubes. The protoplasts of two conjugating cells 

 pass into one of these tubes and there fuse, producing a heavy-walled 

 zygote (Fig. 38C). Upon germination, four cells are formed. Three of 

 these die, the fourth producing a new filament. It is probable that the 

 chromosome reduction takes place when the zygote germinates. 



Spirogyra. Spirogyra is a well-known green alga very common in 

 ponds, lakes, and streams, where it forms slimy bright green masses on or 



