PTEKIDOPHYTES 311 



Chromosome reduction. Reduction of the chromosomes takes 

 place in ferns in the first divisions of the mother cells, so that 

 the spores have one half as many chromosomes in their nuclei 

 as the nuclei of the mother cells (consult Fig. 43, 5). The nuclear 

 division which follows to form cells of the gametophyte and the 

 gametes is of the ordinary vegetative type (Fig. 43, a), in which 

 each daughter nucleus is supplied with the same number of chro- 

 mosomes as the nucleus from which it arose. As a result all 

 cells of the gametophyte, including the gametes, have the re- 

 duced number of chromosomes. When fertilization takes place, 

 the nucleus of the zygote has the two sets of chromosomes sup- 

 plied by the male and female gametes (consult Fig. 44), and 

 this double number persists throughout all cellular divisions in 

 the sporophyte plant until the mother cells of the spores divide 

 to form the first two cells of the tetrads. The two alternating 

 generations in the life history of the ferns are therefore dis- 

 tinct as regards the number of chromosomes in the nuclei of their 

 constituent cells, the gametophyte and the gametes always having 

 one half the number of chromosomes of the zygote, the embryo, 

 and the adult sporophyte, as in the higher plants (Fig. 44). 

 The chromosome changes in the life history, as noted above, 

 always take place in the first division of a spore mother cell to 

 form tetrads and in the formation of the double cell, or zygote, 

 which results from the union of the gametes. The great differ- 

 ences in the form and structure of the two alternating generations 

 in the fern are therefore paralleled by a striking difference in the 

 chromosome number in each generation. 



LIFE HISTORIES OF THE FERN ALLIES (EQUISETALES 

 AND LYCOPODIALES) 



The spore plants of the Uquisetales and Lycopodiales differ 

 so much from the true ferns in external appearance and struc- 

 ture that they would never be recognized as fern allies by the 

 ordinary observer. This alliance was first recognized by botanists 

 after Hofmeister, in 1851, had worked out the life histories of 

 some of the higher spore and seed plants and had established 



