462 



BOTANY 



PART II 



and divides these (2, 3) simultaneously with the other nucleus of the pair. A 

 transverse wall then forms just beneath the protrusion. The upper nucleus 

 from the latter passes into the terminal cell of the filament, while the lower 



protrusion. This 

 off from the 

 a wall and 

 beneath into 



FIG. 411. Armillaria mucida. Clamp formation and 

 development of the basidium. 1, Commencement 

 of clamp formation in the binucleate terminal cell. 

 2, One nucleus passing into the protrusion. 3, 

 Conjugate nuclear division. l+, Clamp -cell and 

 stalk-cell separated from the young basidium. 5, 

 Fusion of the two nuclei. 6, Basidium with single 

 nucleus resulting from fusion. 7, Young basidium 

 with the four basidiospore nuclei and the developing 

 sterigmata. (After H. KNIEP.) 



remains in the 

 then becomes cut 

 terminal cell by 

 fuses with the cell 

 which the nucleus passes. By 

 means of this clamp connection 

 each of the two cells thus obtains 

 a pair of nuclei derived from the 

 original pair. It is possible that 

 the significance of this round- 

 about process lies in its ensuring 

 the distribution of the sister nuclei 

 to the two cells. The binucleate 

 terminal cell gives rise to the 

 basidium. The two nuclei fuse 

 with one another and the resulting 

 nucleus divides to give rise to the 

 four nuclei for the spores (Fig. 411, 

 5, 6, 7). 



This clamp formation corre- 

 sponds, according to KNIEP, to the 

 hook-shape assumed in the develop- 

 ing ascus of many Ascomycetes ; 

 this is, however, limited to the 

 ascogenous hyphae. Both groups 

 contain forms without such arrange- 

 ments, the development of, the 

 basidium or the ascus proceeding 

 directly from the binucleate ter- 

 minal cell of a hypha. 



In the case of some Basidio- 

 mycetes the nucleus of the basidio- 

 spore divides into two and the 

 mycelium with binucleate cells 

 proceeds directly from this without 

 any clamp formation. The genus 

 Hypochnus behaves in this simpler 

 fashion according to KNIEP. 



The binucleate mycelium re- 

 presents the diploid phase. The 



haploid stage commences in the 

 basidium. Its end is indicated by the commencement of clamp connections, 

 but in Hypochnus it is limited to the uiiinucleate stage of the basidiospore. As 

 a result of the suppression of the sexual organs an alternation of generations is 

 no longer present. It can at most be inferred from a phylogenetic point of view. 



Most of the Hymenomycetes develop their profusely-branched mycelium 

 in the humus soil of forests, in decaying wood, or on dying tree trunks, and 

 produce fructifications, commonly known as toadstools, protruding from the 



