SPIROGYRA. 43 



difficulty. In some species of the closely related genus 

 Zygnema, however, the sheath is readily discernible. 

 The increase in the number of cells is effected in the 

 same manner as in the other filamentous plants, i.e. 

 by the division of the cell into halves by a transverse 

 partition always in the same plane, with subsequent ex- 

 pansion of the new cells. 



The distribution of the protoplasm here as in 

 Cladophora and Ulothrix shows a marked advancement 

 over the lower plants. Instead of being diffused evenly 

 through the cell, it forms a layer lining the cell-wall, 

 while it only partly occupies the central portion of the 

 cell. The remaining space is filled by the cell-sap, 

 which consists of water holding various substances in 

 solution. Within the central part of this sap region is 

 the nucleus suspended by means of cytoplasmic threads 

 from the peripheral protoplasm. In the form of the 

 chloroplast band we have a striking feature; for although 

 it is common to have the chlorophyll held in well-defined 

 bodies, it is only in Spirogyra and its close relatives that 

 they assume such peculiar and beautiful shapes. 



The presence of starch granules in the chlorophyll 

 bodies is a very significant fact physiologically. Starch- 

 like substances, which afterwards may be made into 

 starch, are among the first products that chemists have 

 been able to determine in the processes of making food 

 material by plants. 1 



The starch is imbedded in the chloroplasts, and is 

 quite distinct from the pyrenoid, although the constancy 



1 Read on "Photosynthesis" and "Construction of Foods by Green 

 Plants," sometimes called assimilation in various texts at hand. 



