40 COMMON POND SCUM. 



same manner as in Oscillaria, i. e. by the division of 

 the cell into halves by a transverse partition always in the 

 same direction, with subsequent expansion of the new 

 cells. 



The disposition of the protoplasm shows a marked 

 advancement over the lower plants. Instead of being 

 diffused evenly through the cell, it forms a layer lining 

 the cell-wall, known to older botanists as the primor- 

 dial utricle, 6 while it only partly occupies the central 

 portion of the cell. The remaining space is rilled by 

 the cell-sap, which consists of water holding various 

 substances in solution. The nucleus and nucleolus, 

 particularly the latter, are remarkably large. In the 

 chlorophyll band we have a unique feature ; for while 

 it is common to have the chlorophyll separated in well 

 defined bodies, it is only in Spirogyra and its close rela- 

 tives that it assumes such peculiar and beautiful shapes. 



The presence of starch granules in the chlorophyll 

 bodies is a very significant fact in the physiological 

 study of plants. They, or very similar substances, 

 are the first products of assimilation," being the material 

 from which the elaborate frame-work of the plant is 

 eventually constructed. Usually the starch when first 

 formed is scattered irregularly through the chlorophyll 

 bodies ; in Spirogyra, however, the principal part is 

 collected in a layer of granules about definite centers 

 forming hollow spheres. Within these spheres is a 

 highly refractive protoplasmic body, the pyrenoid. 



5 So named by H. v. Mohl, Bot. Zeit, 1844, p. 273 ; The Vegetable 

 Cell, p. 36- 



6 Cf. Sachs , Handbuch d. Exper.-Phys., p. 307 ; Textbook of Botany, 

 2nd Eng. ed., p. 703 ; Bessey, Botany, p. 178. 



