34 BOTANY. 



each division having a single large chloroplast of peculiar form. The 

 central part of the cell in which the nucleus lies is colorless. 



Among the commonest forms, often growing with Spirogyra, are 

 various species of Closterium (Fig. 20, A, B), recognizable at once by 

 their crescent shape. The cell appears bright green, except at the ends 

 and in the middle. The large chloroplast in each half is composed of six 

 longitudinal plates, united at the axis of the cell. Several large pyrenoids 

 are always found, often forming a regular line through the central axis. 

 At each end of the cell is a vacuole containing small granules that show 

 an active dancing movement. 



The desmids often have the power of movement, swimming 

 or creeping slowly over the slide as we examine them, but the 

 mechanism of these movements is still doubtful. 



In their reproduction they closely resemble the pond scums. 



ORDER IV. Siphonece. 



The SiphoneoR are algae occurring both in fresh and salt 

 water, and are distinguished from other algae by having the 

 form of a tube, undivided by partition walls, except when re- 

 production occurs. The only common representatives of the 

 order in fresh water are those belonging to the genus Vauclieria, 

 but these are to be had almost everywhere. They usually 

 occur in shallow ditches and ponds, growing on the bottom, or 

 not infrequently becoming free, and floating where the water 

 is deeper. They form large, dark green, felted masses, and are 

 sometimes known as " green felts." Some species grow also on 

 the wet ground about springs. An examination of one of the 

 masses shows it to be made up of closely matted, hair-like 

 threads, each of which is an individual plant. 



In transferring the plants to the slide for microscopic examination, they 

 must be handled very carefully, as they are very easily injured. Each 

 thread is a long tube, branching sometimes, but not divided into cells 

 as in Spirogyra or Cladophora. If we follow it to the tip, the contents 

 here will be found to be denser, this being the growing point. By careful 



