68 



Practical Plant Biology. 



With the higher objective Spirogyra reveals further beauty. 

 Careful focussing shows that each filament is made up of cylindri- 

 cal cells. The first thing that will impress you is the magnificent 

 colour of the bands in the cells. Focussing clearly demonstrates 

 that each band passes round the central cavity of the cell in a 

 spiral path, within and very close to the cell-wall. If the dia- 

 phragm of the microscope is partly closed this spiral chloroplast 

 will be seen to be embedded in a thin layer of protoplasm lying 

 against, or lining, the inner surface of the cell-wall. The cell- 





FIG. ii. Spirogyra porticalis, x 250. A, focussed on upper side; B, 

 focussed for median plane ; C, focussed for lower side ; D, reconstructed 

 transverse section. 



wall is dazzlingly clear and transparent. It is quite colourless and 

 very thin, as may be seen by focussing for the median plane of 

 the cell. Often the outer surface of the cell-wall is ill defined, the 

 outer layers being less refringent than the inner ones ; they evi- 

 dently form the mucilaginous substance which makes the fila- 

 ments slippery to the touch. The protoplasm of the cell is also 

 transparent but it is finely granular and occasionally the layer 

 immediately in contact with the cell-wall may be seen in circula- 

 tion, reminding one of the protoplasm of Elodea. In this case, 

 however, it is only the layer between the chloroplasts and the wall 



