Metaphyta Spirogyra. 



77 



FlG. 19. Spirogyra longata. 



We commence our study of the Metaphyta, or multi- 

 cellular plants, by examining a plant which exhibits very 

 little morphological differentiation. As an example, we take 

 the common pond weed known 

 as Spirogyra longata. In struc- 

 ture Spirogyra has the form of 

 a green thread composed of a 

 variable number of cells, placed 

 end to end. The cells are all 

 exactly alike, save for certain 

 slight variations in size. Such a 

 collection of more or less simi- 

 lar cells is termed a thallus, a 

 flattened or thread-like cellular 

 expansion. In some cases the 

 thallus may be large, in other 

 cases small. For example, the 

 familiar seaweeds on the coast 

 are instances of large flattened 

 thalli, while Spirogyra itself is an 

 instance of a delicate, narrow, 

 and thread-like thallus. 



We shall see in the present 

 chapter that thalli differ very 

 greatly in form and size. Each 

 cell is composed of a cellulose 

 coat, or cell-wall, with contents. 

 The contents are granular proto- 

 plasm, a nucleus, a green spiral 

 band, and a variable quantity of 

 a watery substance known as cell 

 sap. 



Further details can be made out only by employing high 

 magnifying powers of the microscope. When that is done 

 it is seen that the protoplasm in the cells, at least at some 



Incipient conjugation, a, pro- 

 cesses approaching each other ; 

 6, processes united. 



