300 THALLOPHYTES 
tive to the ordinary cells of the filament; and, when favorable 
conditions for growth return, the protoplast of the resting cell 
breaks through the heavy wall and develops a new filament. 
In Rivularia (Fig. 258), another filamentous form, the filament 
is apparently differentiated into a basal and apical region. A 
heterocyst is the basal cell and the cells decrease in size toward 
the apex, so that the filament has a whip-like 
appearance. 
Besides the features just mentioned in con- 
nection with the plant body, there are some 
other minor ones which some particular species 
of Blue-green Algae have. For example, in 
one species the cells of the colony arrange 
themselves so as to maintain a regular rec- 
tangle. In some forms the colony forms a 
branched filament. 
Food is manufactured, and water and min- 
eral matters are absorbed by these simple 
plants in essentially the same way as in the 
more complex plants, but each cell must 
manufacture food and absorb water and 
mineral matters for itself. Since these plants 
Ste: 208 — 3 dived ter or on a moist substratum, th 
single colony of pe eee ree ; pes 
Rivularia consist. 2%¢ able to absorb water and mineral matters 
ing of a large hete. from their immediate surroundings. Having 
rocyst and many chlorophyll, they are able to carry on photo- 
vegetative cells synthesis and thereby provide themselves 
which decrease in - : 
he Agee trone-thia with carbohydrates. Although the function 
heterocyst. x 540, Of phycocyanin is not known, it is probable 
that it assists some in connection with photo- 
synthesis. Sometimes there is an additional reddish pigment 
developed, which may have something to do with enabling the 
plant to utilize the sun’s rays in the manufacture of food. The 
reddish pigment is so abundant in a few forms that the plants 
appear red in mass, as in one group which forms floating colonies 
in salt water and has given the name to the Red Sea. 
Reproduction in the Blue-green Algae is chiefly by cell division. 
They form no sex cells and, therefore, depend entirely upon 
vegetative methods of reproduction. By cell division new cells 
are formed, which may, according to the species, separate as new 
