LIFE CYCLFS, SFXUAl.ll V, AM) SI XUAF iMFCHANISIVIS 



65 



groups of plants, but such groupings have very little correlation with 

 the major phylogcnctic groupings. The four basic types are: 



(a) Gametic copulatioii, in wliich the two elements brought 

 together in the sexual act comprise uninucleate, free gametes of 

 M'hicli both, one, or neither may be motile. 



(b) Gainete-gainctangiciJ copulation, in which one fusing ele- 

 ment is a differentiated uninucleate gamete and the other is a differ- 

 entiated s^ametangium which produces no discrete, uninucleate 

 gametes. The differentiated gametes may be either 5 or 5 depending 

 upon the group. 





V/ r . 



Fig. 6. The four basic modes of sexual fusion in fungi. 



(a) Gametic copulation, AUomyces arbiisciila. iMotile gametes originating in $ 

 gametangium (above) and $ gametangium fuse to form a zygote that germinates 

 directly to produce a diploid plant (lower right), (b) Gamete-gametangial copulation, 

 Acblya aii?bisex7ialis. Uninucleate gametes, or eggs, in spherical 2 gametangium are 

 fertilized by $ nuclei transferred through tiny tubes from S gametangia; mature, 

 fertilized eggs shown below, (c) Gametangial copulation, Fhycornyces blakesleeamts. 

 A pair of multinucleate gametangia, produced at the tips of large, arched processes, 

 fuse to form a heavy-walled zygospore surrounded by spines, (d) Somatic copulation, 

 Scbizophylhmi coimnime (schematic). Two types of hyphal fusion, tip-to-tip and 

 tip-to-peg, are shown in two stages of development, in an early stage of mutual 

 chemotropic attraction at the left and shortly after fusion at the right. The exchange 

 of nuclei in somatic copulation is typically reciprocal, each mate fertilizing the 

 other. 



(c) Gavietmigial copulation, in which both fusing sexual ele- 

 ments are differentiated as gametangia; one or many pairs of nuclei 

 may be involved, and the two gametangia may be differentiated as 

 $ and 9 or they may be morphologically indistinguishable. 



(d) Soviatic copulation, in which fusion occurs between undif- 

 ferentiated vegetative cells; nuclear migration here is frequently 

 reciprocal, each mate fertilizing the other, and the two compatible 

 nuclei usually retain, once brought together, a dicaryotic association 

 for an indefinite period prior to nuclear fusion. 



