[Chap. XLVII THE ALGAE 633 



division, and all other cells of the plant have the diploid number of 

 chromosomes. 



The reproductive organs are produced in special branches. Swollen 

 conical structures (receptacles) at the tips of the branches contain 

 rounded cavities (conceptacles) in which antheridia and oogonia de- 

 velop. Usually 8 eggs are formed in a single oogonium, and 64 sperms in 

 a single antheridium. Upon release from the oogonium and conceptacle, 

 the non-flagellate egg may be fertilized by a motile sperm. A new plant 

 grows directly from the zygote. Reduction division occurs in the mother 

 cell ( oocyte ) of the 8 eggs and in the mother cell ( spermatocyte ) of the 

 64 sperms. 



The Red Algae: Rhodophyceae 



The red algae are by far the most beautiful of all macroscopic sub- 

 merged plants (Fig. 290). They are most abundant in salt water, but 

 are not infrequent in fresh water. Of the approximately 3500 known 

 species, 200 grow in fresh water. They attain their greatest development 

 in subtropical seas. Five different pigments (Table 17) are known to 

 occur in the cells, with the red usually predominating, although various 

 shades of blue and purple are common. The red algae have definite 

 nuclei and chloroplasts, and the chief accumulated food is a starch-like 

 carbohydrate. They attain greater size on the whole than either the 

 greens, blue-greens, diatoms, or yellow-greens, but none attains the size 

 of the brown kelps. 



The red algae grow attached to rocks near shore, but considerable 

 numbers may be found at depths of 50 to 100 feet, and a few live 200 

 feet or more below the surface. Some species are epiphytes upon other 

 algae. The smallest species are little more than one-celled structures, but 

 the majority are colonial aggregates, or filamentous and much branched, 

 or blade-like and leatherv. The larger species branch profusely, mav at- 

 tain lengths of several feet, and often have considerable differentiation 

 of tissues. A single filament, for example, mav have a row of axial cells 

 ensheathed by a cortical layer several cells in thickness. The red algae 

 superficially resemble other modern algae, but their genealogical rela- 

 tions are very remote. 



Some of the red algae in tropical waters take part in the formation of 

 coral reefs, and are known as coralline algae (Fig. 291). Corals are 

 colonial animals; but always associated with them are red algae that also 

 accumulate calcium carbonate and likewise form massive reefs. Fossil 



