726 THE AMERIC.W .\ A Tl R A 1 .1 ST . XXXVIIl. 



The first mitosis presents half the number of chromosomes 

 fomid in the last nuclear division in the archcsporium and is 

 consequently the reduced or -ametophxte number. The reduc- 

 tion of the chromosomes then takes place durini;- the jjeriod of 

 rest between the last mitosis in the arche>p(n-ium and the first 

 in the spore mother-cell. There are two mitoses in the s]x)re 

 mother-cell. In some forms these are exactly alike and present 

 essentially the same characters as the usual ty[)ical mitoses of 

 plants. Hut among the spermatophytes there are likely to be 

 peculiarities in the arran<rcment and distribution of the chromo- 

 somes. In consequence the first mitosis ma\- be heterotypic 

 and the second homotypic in contrast to the normal tyi)ical con- 

 ditions. The description and explanation of these characters 

 will be reserved for the gi'oups that illustrate them the best. 

 They have nothing to do with qualitati\o reduction phenomena 

 as was formerly supposed. 



There is sometimes a well defined period of rest after the first 

 mitosis with the formation of a wall between the two daughter 

 nuclei, but frequently .the second mitosis follows immediately 

 after the first so that the spore mother-cell comes to contain 

 four daughter nuclei. Cell walls may then be formed between 

 these nuclei simultaneou.sly so that the resultant spores are dis- 

 posed in a radially .symmetrical arrangement that is termed 

 tripartite. These cell divisions are almost utiivei sally present in 

 the spore mother-cell, the only exceptions being certain sperma- 

 tophytes whose megaspore mother-cells dexelop directly into 

 embryo sacs, the two mitoses (heterot) i)ic and homotypic) being 

 included within these structures and tomiing a part of the game- 

 tophyte history. Why the number of sp/nes should generally 

 be four is unexplained. There docs not seem to be any physio- 

 logical significance in the number or other reasons why it should 

 not be more or less. Indeed it is somewhat variable in the 

 spermatophytes for microspore or pollen mother-cells form two 

 and three pollen grains in certain types and five, six and seven 

 have been found in others, while much larger numbers have 

 beer cca nilU j rted. In no case is the microspore 

 mother-cell known to develop directly into a pollen grain, al- 



