DEVELOPMENT OF PLANTS 



215 



at which time the number of chromosomes is (loul^lfd, and, 

 secondly, when the spore mother cell of the asexual Kt-neration 

 divides to form the four spores, tlie number of cliromosomes is 

 reduced by one-half. The two generations arc generally not 

 apparent in the Green and Brown Algae, because the asexual 

 generation is greatly reduced and often does not produce spores, 

 as in Vaticheria, etc., where the asexual generation consists of a 

 single cell, the gametospore. This gamctospore, however, must 

 have twice the number of chromosomes as the mother j^lant, 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 germi- 

 nation of the gametospore results in the reduction of the chromo- 

 somes 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 Ciell divides to form the tetraspores (Fig. 128) 

 and the same reduction has been reported in Spirogyra in the 

 germination of the gametospore. The red algae give us the 

 best illustration of this relationship yet seen. Here the gameto- 

 spore 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 generation, as all its cells contain 

 the double number of chromosomes. The spores from tlie 

 cystocarp form plants externally similar to the sexual plants, 

 but these plants are really different, being characterized b\- cells 

 with the double number of chromosomes and also l)>- tiie fact 

 that they bear tetraspores. The formation of the telrasiwrcs, 

 however, is attended with a reduction of the chromosomes by 

 one-half, and these spores produce plants characterized by the 

 reduced number of chromosomes and also by ilu- tact ili.it they 

 bear the sexual organs. So in the Red Algae we have very 

 clearly brought before us the two generations in ilu- life of the 



