314 A TEXTBOOK OF GENERAL BOTANY 
reproduction is often due to the breaking up of the filament into 
short pieces called hormogonia, which are often limited in size by 
heterocysts. Reproduction may also be due to the formation of 
resting spores. A spore is a one-celled 
| reproductive structure. In all cases 
@: the resting spores of the Cyanophyceae 

C arise from ordinary cells, which gener- 
.) ally increase in size and become largely 
le filled with stored food materials and 
©) surrounded by two distinct membranes. 
> Qe “B® Distribution. The Cyanophyceae are 
6, | (®) found in all parts of the world. They 
| ™ . £ usually occur in 
water or in damp 
localities, but also 
occur in dry situ- 

Fig.315. 
toc with heterocysts. (x 695) ations. ‘They are 
frequently very 
abundant in fresh water and sometimes 
occur in such quantities as to produce a 
distinct color, the so-called ““ water blume.”’ 
Cattle may be killed by drinking such 
water. When the Cyanophyceae occur in 
water in great quantities, their death and 
decay frequently gives the water a very 
unpleasant odor and taste. 
Relationship of Cyanophyceae. The ap- 
parently simple structure of the cell of the 
Cyanophyceae and the absence of a sexual 
method of reproduction indicate that they 
are very primitive forms of plants. The 
bacteria, which lack chlorophyll, are the 
only other plants that do not have definite nuclei, while the 
Cyanophyceae are the only plants with chlorophyll that do not 
have chloroplastids. For these reasons the Cyanophyceae are 
regarded as the most primitive chlorophyll-bearing plants. 

Fra.316. Anabaena with 
heterocysts. (x 535) 
