STRUCTURE OF THE PLAXT-CELL 7 



chloroplasts, essentially similar in form to those just studied. 

 In the simple tj'pes of plant-life [e.g. among the Alga:-), however, 

 the cells are often provided with but a single chloroplast or a 

 Umited number of them, and these are frequently of a much 

 more elaborate pattern. Spirogyra, which is exceedingly common 

 in stagnant water, furnishes an extreme instance. The plant 



Fig. 4. — Single cell of a Spirogyra fdameat showing the spiral chloroplast 

 (ch.), the pyrenoids {p.) surrounded by starch, and the nucleus (».) 

 supported by cytoplasmic strands [si.). S, separating wall between 

 adjacent cells. 



here consists of a single thread or filament composed of cylin- 

 drical cells joined end to end. According to the species, each 

 cell contains, apart from the cytoplasm and nucleus, one or 

 more chloroplasts in the shape of green spiral bands, usually 

 with a jagged edge (Fig. 4, ch.). These spiral chloroplasts are 

 here, again, situated in the lining la3-er of cj'toplasm. At inter^-als 

 bright bodies, consisting of protein ancl known as pyrenoids, are 



