52 



PLANT MORPIIOLOG Y 



beneath the surface of the water. It is a large genus of over 100 species. 

 The vegetative bod}^ is an unl)ran('hed filament with cylindrical cells that 

 are usually elongated. Each cell has a single nucleus suspended in the 

 center by strands of cytoplasm (Fig. 39.4). It also has one or more 

 peripheral, ribbon-like chloroplasts with many pyrenoids. The chloro- 

 plasts have the form of spiral bands, the number in each cell depending 



G H 



Fig. 40. Nuclear changes in the zygote of Spirogyra longata {A to G) and germination of the 

 zygote of Spirogyra neglecta (H). A, B, C, first meiotic division of fusion nucleus in the 

 zygote; D and E, second division; F and G, degeneration of three of the haploid nuclei. 

 (After Trondle.) 



on the species. Any cell may divide by the formation of a cross wall, thus 

 resulting in growth of the filament. In some species the cross walls 

 possess characteristic infoldings. 



In sexual reproduction the cells of the two filaments lying side by side 

 put out lateral projections that come in contact (Fig. 395-Z)). The 

 contiguous portions of the cell walls at the ends of these projections then 

 break down and form tubes leading from one filament to the other. 

 Through these conjugating tubes the protoplasts of one filament pass to 

 fuse with those of the other filament, forming zygotes. An entire vege- 

 tative protoplast thus becomes a large gamete. The zygote develops a 

 heavy wall and goes into a resting stage. Upon germination, which 

 usually occurs in the following spring, it directly produces a new filament 



