i68 REPRODUCTION 



hyphae that envelope the first stages of the carpogonia. It is vegetative in 

 origin and distinct from the generative system. 



In lichens belonging to the Collemaceae, the paraphyses rise from the 

 branching of the carpogonial stalk-cell immediately below the ascogonium', 

 but have no plasma connection with it. They are thus comparable in origin 

 with the paraphyses of many Discomycetes. 



In several genera in which the algal constituents are blue-green, such as 

 Stictina, Pannaria, Nephroma, Ricasolia and Peltigera, Sturgis" found that 

 reproduction was apogamous and also that asci and paraphyses originated 

 from the same cell-system : a tuft of paraphyses arose from the basal cell 

 of the ascus, or an ascus from the basal cell of a paraphysis. These results 

 are at variance with those of most other workers, but the iigures drawn by 

 Sturgis seem to be clear and convincing. 



Again in Usnea barbata, as described by Nienburg', the ascogonial cells, 

 after the disappearance of the trichogyne, branch profusely not only up- 

 wards towards the cortex but also downwards and to each side The upward 

 branches give rise normally to the asci, the lower branches produce the sub- 

 hymenium and later the paraphyses, and the two systems are thus genetically 

 connected, though they remain distinct from each other, and asci are never 

 formed from the lower cells. 



In most heteromerous lichens, however, the origin of the paraphyses is 

 exclusively vegetative : they arise as branches from the primordial complex 

 that forms the covering hyphae of the ascogonium both above and below. 

 Schwendener^ had already pointed out the difference in origin between the 

 two constituents of the hymenium in one of his earlier studies on the de- 

 velopment of the apothecium, and this view has been repeatedly confirmed 

 by recent workers, except by Wahlberg' who has insisted that they rise fronj 

 the same cells as the asci, a statement disproved by Baur*. The paraphyses 

 originate not only from the covering hyphae, but from vegetative cells in 

 close connection with the primordium. In this mode of development, lichens 

 diverge from fungi, but even in these a vegetative origin for the paraphyses 

 has been pointed out in Lachnea scutellata' where they branch from the 

 hyphae lying round the ascogonium. 



There is no general rule for the order of development. In Lecanora sub- 

 fusca Baur" found that vertical filaments had reached the surface by the time 

 the trichogyne was formed, and their pointed brown tips gave a ready clue 

 to the position of the carpogonia. In Lecidea enteroleuca^ they show their 

 characteristic form and arrangement before there is any trace of ascus 

 formation. In Solorina^ they are well formed before the ascogenous 

 hyphae appear. In other lichens such as Placodium saxicolum", Peltigera 



' Baur 1899. ^ Sturgis 1890. ' Nienburg 1908. ^ Schwendener 1864. * Wahlberg 1902. 

 * Baur 1904. ' Brown 1911. 8 Moreau 1916. " Lindau 1888. 



