334 A TEXTBOOK OF GENERAL BOTANY 
egg surrounds itself with a thick wall and becomes a resting 
spore. After a period of rest it develops directly into a new plant. 

Fig. 344. Halimeda opuntia, a calcareous 
green alga 
Relationship. The Siphonales, the 
order to which Vaucheria belongs, are 
a branch of the Chlorophyceae which is 
the result of a line of evolution very : : 
different from the main line that gave oe al 
rise to higher green plants. Since we 
regard flowering plants as a’ culmina- 
tion of the evolutionary process in 
plants, the order Stphonales represents a side branch of evolu- 
tion. One of the subclasses of fungi is believed to have been 
derived from this order. 

Fig. 345. A cell of Spiro- 
gyra. (xX 425) 
S'PIROGYRA 
Cell structure. Spirogyra is a filament composed of cells joined 
end to end, all of the cells being similar in structure. Each cell 
contains one or more chloroplastids which have the form of spiral 
bands (Fig. 345). In each chloroplastid there is a longitudinal 
