480 BOTANY OF THE LIVING PLANT 



in Animals, in which the whole body appears to be consistently 

 diploid. Thus while Botanists have assumed the term "Alternation 

 of Generations " first used in relation to Animals, they now apply 

 it to a phenomenon in Plants which proves to be peculiar to them. 

 The descriptions already given of the life-histories of Plants have 

 provided many facts which may now be drawn together into a 

 comprehensive statement on Alternation, and on the changes and 

 modifications which it shows in relation to habit. 



In normal Plant-Organisms which show sexuality the fusion of two 

 nuclei in syngamy has been found to result in a doubling of the number 

 of chromosomes in the zygote. This has been demonstrated in so 

 many well-authenticated cases that it may be held as a general con- 

 sequence. At some other point in their life-cycle, and always before 

 another sexual fusion occurs, there is a tetrad-division, which results 

 in the reduction of the number of chromosomes again to one half. 

 The second process may be held to be complementary to the first, 

 and it appears to be necessary if the number of chromosomes is to 

 be kept within limits after successive generations. The life-cycle in 

 sexually propagated plants is thus made up of two phases : the one 

 intervenes between syngamy and reduction, and is diploid, i.e. with 

 2x chromosomes. It is commonly styled the sporophyte, or non- 



REDUCTION. SYNGAMY 



sexual generation, because it usually terminates in the production 

 of non-sexual spores. These spores are consequent on a tetrad- 

 division, and may be styled specifically carpospores. The other is 

 haploid, i.e. with x chromosomes. It is commonly styled the gameto- 

 phyte, or sexual generation, because it normally results in the forma- 

 tion of gametes. The cycle thus constructed may be represented as in 

 the diagram given above. Since the two phases follow one another in 



