266 A TEXTBOOK OF THEORETICAL BOTANY 



said to be sessile, or they may be elevated on a short stalk. Externally they 

 are white or buff in colour, but the inside of the cup is brown. The outer 

 surface is frequently covered by minute wart-like pustules, but the inside 

 is smooth. 



Asexual Reproduction 



This type of reproduction occurs irregularly by the production of 

 conidiospores, or by the formation of thick-walled spores, termed chlamydo- 

 spores, which develop singly or in groups within the cells of the mycelium. 

 Both germinate to produce fresh hyphae. 



Sexual Reproduction 



Sexual reproduction in the Pezizales varies considerably. A series of 

 types showing progressive reduction in the sex organs can be seen by a 

 comparison of species. Firstly it appears the antheridium ceases to function, 

 as we have already seen in certain species of Aspergillus. Then the trichogyne 

 of the oogonium disappears, and finally the oogonium itself is not formed, 

 but in its place certain cells of the vegetative hyphae take over its function 

 and all that is left of the original sexual process is the fusion of two nuclei 

 within a purely vegetative cell. 



In Peziza vesiculosa the apothecium begins as a weft of tangled hyphae, 

 and in the centre of this mass are certain cells which are said to possess 

 nuclei which fuse in pairs. These cells then give rise to ascogenous hyphae. 

 From these the asci are formed, and in them the ascospores are produced. 

 There exists in this species the same question as to whether there are two 

 fusions, the one of the paired nuclei in those vegetative cells which play the 

 part of the sex organs, and the second fusion in the ascogenous hyphae, or 

 whether there is only the second one. According to which view is adopted 

 the last division of the nuclei in the formation of the ascospores will either 

 be regarded as a reduction division or not. The ascospores are arranged 

 obliquely in a row within the ascus, and each is ellipsoidal and smooth 

 in outline. 



The asci stand side by side forming a continuous layer over the inner 

 surface of the apothecium. They are cylindrical and are intermingled with 

 straight, slender hyphae, which are termed paraphyses. The special layer 

 formed of the asci together with the paraphyses is termed the hymenium 

 (Fig. 258). The eight ascospores are discharged from the ascus through a 

 terminal aperture, only under damp conditions. Breathing on the surface 

 of a ripe fruiting body will often cause the liberation of a cloud of spores. 

 On germination they either produce a fresh mycelium, or they may give rise 

 to a hypha from which conidia are developed. 



Helvellales 



The Helvellales are large Ascomycetes usually growing saprophytically 

 on dead wood, in which the fruiting body is fleshy and in which the hymenium 



