CHAPTER XXI 

 ALTERNATION OF GENERATIONS 



Throughout the book we have been repeating that the micro- 

 spore is the first cell of the male gametophyte, the megaspore is the 

 first cell of the female gametophyte, and that the fertilized egg is the 

 first cell of the sporophyte. This may seem trite, but it is funda- 

 mental and the evolution of these structures in the gymnosperms is 

 so suggestive that it seems worth while to consider them in relation 

 to those which are lower and higher in the scale. 



That the gametophyte and sporophyte generations alternate in 

 seed plants is accepted by every botanist. Similar alternation is just 

 as readily recognized in the pteridophytes and bryophytes; but, in 

 the thallophytes, there is not the same unanimity of opinion, partly 

 because the average botanist is not so famihar with life-histories in 

 thallophytes and partly from failure to recognize the origin of alter- 

 nation. 



Below the level of sexuality there is no alternation of generations: 

 merely cell division. A cell may grow larger than its neighbors, 

 become surrounded by a thicker wall, and tide the plant over un- 

 favorable conditions; but there is no alternation. If such a plant 

 has differentiated chromosomes, we should regard their number as 

 the X number. 



When a plant reaches the level of sexuality, two cells (or their 

 nuclei) fuse. When they fuse, each contributes the x number of 

 chromosomes and the number is doubled, so that the resulting cell, 

 the zygote, has 2x chromosomes. 



At first, the fusion was probably only a stimulus to development, 

 and a reduction of chromosomes occurred at the first two mitoses in 

 the zygote. No plant body, except the zygote, was built up. Spiro- 

 gyra and many of the Chlorophyceae show this condition, and many 

 botanists do not recognize any alternation; but alternation has orig- 

 inated, and, from this beginning, the evolution of the ix generation 



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