224 



GENERAL BOTANY 



The chloroplast is by far the most striking structure in the 

 cells of Spirogyra, when viewed with a compound microscope. 

 These chloroplasts are in the form of bands, one or more in each 

 cell (Fig. 117), wound spirally around the cell just beneath the 

 cell wall. Each chloroplast is in reality a differentiated portion 

 of the cytoplasmic sac and corresponds, therefore, in origin and 



substance, to the granular plas- 

 tids of higher plants. In sections 

 of Spirogyra cells (Fig. 118) 

 these bandlike chloroplasts are 

 seen to be thickened and to 

 project as ridges of cytoplasm 

 into the cell vacuole slightly 

 more than the intervening por- 

 tions of the sac. 



Under proper magnification 

 each chloroplast band is also 

 seen to be dotted at regular 

 intervals with a series of disk- 



-P 



n 



FIG. 117. Cell structure of Spirogyru 



A, single cell of Spirogyra with nucleus 

 (ri) and spiral chloroplast containing 

 pyrenoids (p) ; B, cell with the cyto- 

 plasmic sac contracted by treatment with 

 salt solution ; C, pyrenoid in a small por- 

 tion of a chloroplast. From Bergen and 

 Davis's " Principles of Botany " 



shaped bodies called pyrenoids, 

 the entire series of pyrenoids 

 being connected by a delicate 

 strand of cytoplasm which is 

 devoid of green pigment. Upon 

 investigation with the high 

 power of the microscope the 

 pyrenoid is found to be differ- 

 entiated into a darker central granule and a lighter surrounding 

 sheath, called the starch sheath. The function of the central 

 granule is not definitely known, but the starch sheath is a 

 storage place for the excess of starch not used by the cell dur- 

 ing the daylight, when the chloroplast is forming sugar and 

 starch by photosynthesis. At night this starch is digested or 

 transformed into sugar and is used for the nutrition of the cells 

 during the hours when photosynthesis is not going on. The 

 starch sheath is therefore a temporary storage structure of the 

 cell and of the chloroplast. The above function of the pyrenoid 



