ALTERNATION OF GENERATIONS 441 



and the reduction of the x generation afford a splendid study in the 

 comparative morphology of plants. If zoologists knew this whole 

 line of evolution and reductions they might find a reasonable in- 

 terpretation for the three polar bodies. 



In many algae reduction does not take place at the germination 

 of the zygote, but there is a more or less extended period of cell 

 division preceding it. In such cases the 2X body becomes large 

 enough to be seen with the naked eye. It may or may not look like 

 the X body. In Zanardinia the x and 2X bodies look alike; but in 

 Cutleria, in the same family, they look different. In the Laminaria- 

 ceae the 2x body has become immensely larger than the x body. 

 In Coleochaete scutata the germinating zygote builds up a body which 

 does not look like the structure that produced the gametes; but this 

 new body, usually consisting of eight cells, has the x number of 

 chromosomes, the reduction taking place during the first two mi- 

 toses in the zygote, as in Spirogyra. It would be interesting to know 

 where reduction takes place in those species of Coleochaete, which 

 have more than eight cells in the body produced by the zygote. As 

 many as sixteen and thirty-two have been counted. All these zy- 

 gotes produce spores. Consequently, the eight-celled body, in 

 Coleochaete, although an x structure, is etymologically a sporophyte. 

 Many x bodies produce spores, and, in this sense, are sporophytes; 

 but the same plant body may produce both spores and gametes, as 

 in Ulothrix. Fucus, a 2X body, produces gametes. When one's 

 theories of alternation are based only upon plants from the bryo- 

 phytes up, the fundamentals of the phenomenon may be over- 

 looked. For this reason, we have reiterated: the spore is the first 

 cell of the gametophyte, and the zygote is the first cell of the sporo- 

 phyte. We might have said x and 2X generations; but from the 

 bryophytes up, and in many thallophytes, x and 2x generations are 

 synonymous with gametophyte and sporophyte generations. When 

 plants which he regards as gametophytes produce spores, and those 

 which he regards as sporophytes produce gametes, some botanists 

 become confused. The trouble is that, in these cases, gametophyte 

 and X generation and sporophyte and 2X generation are not synony- 

 mous. The terms "gametophyte" and "sporophyte" are not as 

 broad as x and 2x generations. 



