214 LIFE HISTORY OF ALGAE 



finally secrete cell walls and develop into new plants. These 

 plants, however, normally bear only gametangia and not tetra- 

 spores. So in this life history we have two similar plants appear- 

 ing but they are manifestly different in nature as one produces 

 oprastetres and the other gametangia. 



The red algae are of considerable economic importance. 

 Irish moss, CJiondrus, is used in the manufacture of jellies, and 

 agar-agar is obtained from several species of algae. Many tons 

 of various kinds of red algae are annually dried and consumed 

 for food in the East. The sw^allow's nest, of which you have 

 heard so much as an article of food in the Orient, is constructed 

 of algae. 



75. Significant Features in Life History of the Algae. — The 

 germination of the gametospore in the Rhodophyceae is a note- 

 worthy departure in two respects. Unlike previous cases, it is 

 retained on the plant, where it is nourished during its germination 

 and grows practically as a parasite. This relation of the gameto- 

 spore to the mother plant will become more noticeable among 

 the fungi and the mosses and lead to pronounced changes in the 

 life history of the plant. In the second place, we notice that the 

 gametospore in the red algae produces a number of cells before 

 the spores are formed. In the green and brown algae it either 

 developed directly into a plant similar to the mother plant, as in 

 Vaucheria and Fucus, or zoospores w^ere first formed w^hich de- 

 veloped into a plant similar to the parent, as in Ulothrix and 

 Oedogonium. What is the significance of this variation in the 

 germination of the gametospore? Attention has been directed 

 in the study of Spirogyra and Ulothrix, p. 193, to the fact that 

 there are two phases in the life history of a plant, a spore-bearing 

 phase or generation, and a sexual phase or generation. This is 

 doubtless true of all forms characterized by a sexual reproduction. 

 On page 136 you have noticed that these two phases or genera- 

 tions are also sharply distinguishable by the number of chromo- 

 somes appearing in the nuclei of the cells, the sexual generation 

 having only half the number found in the asexual. This change 

 in the number of chromosomes occurs at two points in the life 

 history, first, when the gametes unite to form the gametospore, 



