i 9 7 
The Cytology of Zygnema ericetorum. 
rare occasions three or four pyrenoids have been seen in one 
chloroplast. These conditions, however, are only irregularities 
since the chloroplast normally contains two pyrenoids. 
Fio. 2. Zygnema ericetorum. A, aquatic form ; B. terrestrial form. Both 
examples fixed and stained as mentioned in text. C, filament of terrestrial 
form producing aplanospores on dying. A and B, x 1627; C, x 813. 
The nucleus is situated in the central region of the cell, being 
lodged in the constriction of the chloroplast. It varies somewhat 
in size, but is mostly very small (cf. Fig. 1 A n). 
Reserve material. In the living alga collected in October and in 
all the examples collected through the winter, the chloroplast is to 
a large extent masked by numerous oil-globules. These globules 
