DEVELOPMENT OF PLANTS 193 



with adjacent cells that contain storage foods. In this way, the 

 formation of a larger number of spores is made possible. Fre- 

 quently a sac-like structure, the cystocarp, is developed about the 

 spores owing to the outgrowths of the adjacent cells (Fig. 128, 

 C). The red algae are of considerable economic importance. 

 Irish moss, CJwndrus, 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 swallow'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 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 di- 

 rectly into a plant similar to the mother plant, as in Spirogyra, 

 Vaucheria and Fucits, or zoospores are first formed which devel- 

 oped into a plant similar to the parent, as in Ulothrix and Oedo- 

 gonium. What is the significance of this variation in the germi- 

 nation of the gametospore? Attention has been directed in the 

 study of Ulothrix, p. 173, 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 122 you 

 have noticed that these two phases or generations are also sharply 

 distinguishable by the number of chromosomes appearing in the 

 nuclei of the cells, the sexual generation having only one 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, at which time the 

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