CYTOLOGY OF REPRODUCTION IN PLANTS 163 



iiniciliate male and female gametes differing in size. The plants devel- 

 oped from the zygotes bear zoospores which j^roduce the next generation 

 of sexual plants. 



The most familiar basidiomycetcs are the rusts, which do so much 

 damage to grain crops, and the mushrooms. The latter will furnish 

 our example of nuclear cytology in this group of fungi (Fig. 120). Th(> 

 spores discharged from the gills or pores of the mushroom germinate and 

 produce a septate primary mycelium, usually \\dth one nucleus per cell 

 but sometimes with more. IMost species are heterothallic, the spores 

 being of two kinds and producing plus and minus mycelia, respectively. 

 A primary mycelium may produce small asexual reproductive bodies 

 known as oidia, but usually it does not develop the familiar sporophores, 

 or mushrooms. When plus and minus mycelia come in contact, their 

 hyphae unite at one or more points where openings are formed in the 

 intervening walls. At each point of union the plus nucleus divides, one 

 of the daughter nuclei then passing into the minus hypha. At the same 

 time, the minus nucleus divides, one of its daughter nuclei passing into 

 the plus hypha. In this way each primary mycelium comes to have a 

 binucleate cell. In the plus hypha the introduced minus nucleus divides, 

 one of the products passing through a pore in the w^all into an adjacent 

 plus cell, rendering it binucleate also. This process is repeated cell by 

 cell until much of the primary plus mycelium becomes diploidized. The 

 binucleate mycelium so formed is called a secondary mycelium. Mean- 

 while the same process may be carried out by the plus nucleus delivered 

 to the minus mycelium so that it, too, becomes diploidized. The two 

 mycelia are thus mutually diploidized. As the binucleate secondary 

 mj'celium continues its growth, the nuclei, now in pairs, divide in unison 

 (conjugately). Each pair of nuclei is called a dikaryon, and this second- 

 ary mycelium, which is the stage commonly observed in nature, is accord- 

 ingly known as the dikai-yophase. It is often said to be diploid, even 

 though the nuclei are individually monoploid, for the protoplasmic 

 activity is now influenced by two genomes differing in constitution. 



Numerous hyphae of the secondary type become much intertwined 

 and differentiated as thick strands (rhizomorphs) which develop the 

 sporophores. At the surface of the gills, or in some species the pores, of 

 the sporophore the ends of some of the binucleate hj'phae enlarge as 

 basidia. In each young basidium are two nuclei which are descendants 

 of the two brought together when primary hyphal union and diploidiza- 

 tion occurred. The two nuclei now fuse (karyogamy), doubling the 

 chromosome number. Then the diploid nucleus undergoes two meiotic 

 divisions, and the four resulting nuclei pass into the four basidiospores 

 which are budded off from the basidium. Wh(^n these spores germinate, 

 two of them produce plus while two produce minus mycelia. This can 



