THE EVOLUTION OF THE PLANT BODY 



27 



The Multicellular Plant Body without Division oj Labor 



In a POND SCUM such as Spirogyra (fig. 9) we find a multicel- 

 lular aquatic plant with no division of labor among the cells 

 which make up the organism. Each filament of the thread-like 

 plant is made up of a number of cells in a single row, attached 

 end to end. Young plants consist of a few cells and may be at- 

 tached to the substratum by the basal cell which acts as a hold- 

 fast. Older plants consist of hundreds of cells, and are generally 

 floating. All of the cells which make up the Spirogyra plant look 



NUCLEUS, 



CHLOROPLAST 



Fig. 9. — In a pond scum such as Spirogyra we find a multicellular aquatic 

 plant with no division of labor among the cells; upper drawing an enlarged 

 portion of the plant seen in lower drawing. 



alike and carry on the same functions, so that for practical pur- 

 poses it is simply a number of individuals — each similar to a 

 desmid cell — ^which have remained united after cell division. The 

 greater part of each cylindrical cell consists of a water vacuole 

 filled with cell sap, a thin layer of cytoplasm separating it from 

 the cell membrane and cell wall; strands of cytoplasm extend 

 inward and surround the central nucleus. This cytoplasmic 

 layer contains one or more band-shaped chloroplasts which 

 form bright green spirals around the cell wall. These carry on 

 photosynthesis in the same manner as the chloroplasts of the 

 desmid. Each cell of the filament independently takes in its 

 own carbon dioxide, oxygen and minerals from the aquatic 



