264 PLEUROCOCCTTS : SPIROGYRA. 



Scrape a little of the green powder from a piece of tree 

 bark on to a slide, mount in water, and note the cells, which 

 are either isolated or, more often, associated in groups or 

 packets. Unless the material has been soaked in water 

 beforehand, it will probably be difficult to see clearly on 

 account of air -bubbles ; to remove these, add a little alcohol. 

 From the appearance of the cell-walls it is easy to see that 

 the packets are temporary aggregates or colonies of cells 

 which do not immediately separate after division and the 

 formation of walls, but which gradually split apart as 

 division continues. In a single cell note (1) the well- 

 marked cell-wall; (2) the chloroplast, which is lobed 

 and perforated so as to present the deceptive appearance 

 of a number of separate chloroplasts ; (3) the colourless 

 vacuolated protoplasm, nucleus; and (4) a pyrenoid 

 (not always present). 



In order to see the parts of the cell more clearly, mount 

 some material in iodine, and some in chlor- zinc-iodine ; 

 also steep some material in alcohol, to remove the chloro- 

 phyll, and then treat it with these reagents. Starch grains 

 will probably be seen in the chloroplast. 



In addition to the simple vegetative multiplication due to the dis- 

 sociation of the loosely connected colonies of cells, reproduction in 

 Pleurococcus is brought about by (1) the formation of resting cells 

 (gonidia) either directly or after a few divisions of the cell, (2) the 

 formation of biciliate zoogonidia, (3) the conjugation of isogamous 

 biciliate zoogametes. 



SPIROGYRA. 



378. Occurrence. One or other of the various species 

 of Spirogyra may be found at almost any time of year, 

 but as a rule they are (at least in the south of England) 

 most abundant in spring and early summer ; plants 

 may be found in the vegetative condition throughout the 

 winter, while conjugation occurs chiefly from April to 

 June. 



Spirogyra grows commonly in low-lying quiet waters, as 

 large flocculent green mats covering the surface in ponds 



