544 BOTANY OF THE LIVING PLANT 



This distinction is not matched by any corresponding known difference 

 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 possess sexuality the fusion of two 

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



REDUCTION. SYNGAMY. 



X 



Fig. 427. 



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, normally before 

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

 in the reduction of the chromosomes again to the original number. 

 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 repeated syngamy. 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- 

 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 tetraspores. 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 formation 



