1G4 



FUNDAMENTALS OF CYTOLOGY 



be determined by removing the quartet of spores from a basidium and 

 cultivating them separately. 



In this reproductive cycle, which is paralleled in structures of very 

 different appearance in the rusts, the stage of greatest cytological interest 

 is the dikaryophase. The significance of this peculiar nuclear state will 

 be pointed out below. 



The ascomycetes, like the basidiomycetes, include forms with cycles 

 including a dikaryophase. The spores, which are borne in sacs, or asci, 



Fig. 120. — Diagram of nuclear history in the Ufe cycle of a heterothallic basidiomycete. 

 At the point of hyphal union the plus and minus nuclei are about to undergo division. 

 One of the products of each division will pass to the other hypha (see text). One primary 

 mycelium is shown partially diploidized as it would be at a later stage; the other is com- 

 monly diploidized also. Diploidization has been followed by the formation of a sporophore. 

 The arrangement of successive stages in the basidia is arbitrary. Natural proportions are 

 not represented, and nuclei are not drawn in all cells. 



produce septate uninucleate mycelia, which in heterothallic species are 

 of two kinds, plus and minus. The uninucleate mycelium in forms like 

 Peziza or Pyronema develops the familiar apothecia, or open cup-shaped 

 fruit bodies (Fig. 121). At a very early stage in the development of the 

 apothecium there differentiates in its midst a multinucleate female organ, 

 the ascogonium, and a multinucleate antheridium. The nuclei of the 

 latter are discharged through a trichogyne into the ascogonium, where 

 they mingle with the nuclei of the ascogonium. The ascogonium then 

 sends out a number of ascogenous hyphae, into which pairs of nuclei 



