212 



PRACTICAL BOTANY 



195. Uses of red algae. Several genera of the red algse are 

 used as food. They may be dried and thus kept for long 



periods. The gelatinous mate- 

 rial that is secured from them 

 forms a delicacy much de- 

 sired by some people. In the 

 North Sea and elsewhere in 

 the Atlantic Ocean occurs a 

 red alga known as "Irish 

 moss," which is collected in 

 large quantities and employed 

 in the preparation of jelly, to 

 be used both directly as food 

 and as the basis for the prep- 

 aration of other foods. One of 

 these gelatinous products of 

 red algae is agar-agar, which is 

 extensively used as a growth 

 medium in bacteriological 

 work, and in similar work 

 with some of the lower fungi. 



FIG. 175. The asexual reproduction 

 of a red alga (Callithamniori) 



At the left there is a branch upon which 

 is a sporangium. Within its wall its di- 

 vision into four spores has taken place. 

 At the right these four asexual spores 

 have escaped from the sporangium. 

 Much magnified. After Thuret 



196. Classification of the algae : 



Thallophytes 

 Algse 



Class I. Chlorophyceae (the green algae) 



Leading genera used as illustrations, Pleurococcus, Spirogyra, 

 Cladophora, Ulothrix, Draparnaldia, Vaucheria, Zygnema, 

 CEdogonium, Chara 

 Class II. Phaeophyceae (the brown algae) 



Leading genera used as illustrations, Fucus, Sargassum, Lami- 



naria, Ectocarpus 

 Class III. Rhodophyceae (the red algae) 



Leading genera used as illustrations, Gigartina, Dasya, Calli- 

 thamnion 



