DEVELOPMENT OF PLANTS 203 



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 129 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 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 

 number of chromosomes is doubled, and, secondly, when the 

 spore mother cell of the asexual generation divides to form the 

 spores, the number of chromosomes is reduced by one-half. 

 The two generations are generally not apparent in the Green and 

 Brown Algae, because the asexual generation is greatly reduced 

 and often does not produce spores, as in Vaucheria, Spirogyra, 

 etc., where the asexual generation consists of a single cell, 

 the gametospore. This gametospore, however, must have twice 

 the number of chromosomes as the mother plant, since it is formed 

 by the union of the chromatic substance of two gametes and it 

 will doubtless be found that its first divisions in germination are 

 attended with a reduction of the chromosomes to the original 

 number found in the mother plant. This remark applies also to 

 Ulothrix and Oedogonium, where it is reasonable to suppose that 

 the formation of the four zoospores by the germination of the 

 gametospore results in the reduction of the chromosomes to the 

 number found in the mother plant. This has been found to be 

 the case by Allen in the division of the germinating gametospore 

 of Coleochaete, and Yamanouchi reports that the reduction of the 

 chromosomes in the Red Algae does not occur until the mother 

 cell divides to form the tetraspores (Fig. 126). For example, in 

 Polysiphonia the gametospore germinates, producing a small 

 spore-bearing plant, enclosed in the cystocarp, which is parasitic 

 upon the sexual plant. This plant is a part of the asexual gen- 

 eration, as all its cells contain the double number of chromosomes. 

 The spores from the cystocarp form plants externally similar to 

 the sexual plants, but these plants are really different, being 

 characterized by cells with the double number of chromosomes 



