i8 



A TEXT-BOOK OF BOTANY. 



ular or Brownian movement. Reproduction is either by simple 

 division or by the union of two Desmids. In the latter case the 

 contents of each flow together into a connecting tube formed by 

 the union of the two Desmids, the resultant mass rounding ofif to 

 form a zygospore. 



Fig. 8. Forms of Desmids in longitudinal view and transverse section. A, 

 Mesotaenium Braunii; B, Ancylonema Nordenskioldii; C, Penium Digitus; D, Cylin- 

 drocystis crassa ; E, Closteriu'm moniliferum; F, Spirotaenia muscicola; G, Pleiiro- 

 taenium Trabecula; H, a Docidiiim Baculum; H, a' D, dilatatum. — From Wille in 

 Engler and Prantl's "Die Natiirlichen Pflanzenfamilien." 



Spirogyra. — Another one of the common Green Algae is 

 Spirogyra (Fig. 9), one of the pond-scums, which in the spring 

 forms floating green masses on ponds and shallow water. The 

 plant-body consists of a chain of cylindrical cells forming long 

 threads or filaments. The transverse walls are sometimes pecu- 

 liarly thickened. The chromatophores occur in one or more spiral 



