THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 273 



cells, now completely disorganized. As these cavities enlarge vertically the 

 conidiophores commence to cut off conidiospores in chains and fill the cavities, 

 which eventually burst open by the breakdown of the upper epidermal walls. 

 Such cavities, in which asexual conidia are produced, are termed acervuli. 

 Despite the fact that these conidiospores are produced in vast numbers they 

 have not been germinated under experimental conditions, and there is no 

 evidence that they do actually spread the Fungus during the summer months 

 when they are mainly produced. 



Sexual Reproduction 



The apothecia may be formed either in old acervuli or in newly formed 

 cavities in the epidermis. During the autumn the plectenchvma becomes more 

 sclerotic and blackens considerably. In the interior of the plectenchyma 

 numerous apothecial cavities become differentiated. These become filled 

 with loose hyphal tissue and are covered not only by the outer epidermal 

 walls but also by a thick-walled black tissue called the epithecium. Below, 

 the cavities rest on a somew^hat lighter, thick- walled tissue, the hypothecium, 

 which in turn covers the lower epidermal walls. 



In each of these apothecial cavities the female sex organs are formed. 

 Each consists of a uninucleate stalk cell, two or three multinucleate ascogonial 

 cells and a uninucleate trichogyne. Apparently no male sex organ is formed 

 and no true fertilization takes place. Instead, the septa between the ascogonial 

 cells break down and their nuclei pair together. 



Ascogenous hyphae now grow out from the ascogonium and paired nuclei 

 migrate into them. The ascogenous hyphae branch repeatedly, and many 

 asci are produced from a single ascogenous hypha. Each voung ascus 

 possesses a pair of nuclei derived by conjugate division from the original 

 pair in the ascogenous hypha. These two nuclei then fuse. 



The single fusion nucleus in the ascus now undergoes three divisions, 

 resulting in the formation of eight nuclei, around which eight ascospores are 

 formed. These ascospores are long and slender and lie side by side in a 

 packet along the length of the ascus. 



Meanwhile, from the loose hyphae in the apothecial cavity paraphyses 

 are differentiated which grow up between the asci, so that at maturity the 

 asci are more or less enveloped in a mass of very slender, elongated filaments. 



The mechanism for bursting the apothecium is elaborate. Changes, 

 mainly of the roof of the apothecial cavity, are initiated about the time that 

 the asci begin to form. A layer of cells about three-quarters of the depth 

 from the top break down, thus forming a small rift which gradually expands 

 downw'ards, while at the same time the upper part of the roof rises and 

 finally ruptures at the top (Fig. 264). At this stage the apothecium is closed 

 only by the tissue below the rift. When the spores are ripe this too ruptures 

 and the apothecial cavity is exposed. Some apothecia never open widely 

 and the opening remains as a narrow fissure (Fig. 265), in others the roof 

 bends back so that the whole cavity is exposed. 

 10 



