174 REPRODUCTION 



walled hyphae, with branching vertical to the surface, and so closely packed 

 that there is an appearance of plectenchyma ; the medullary hyphae are 

 also thick-walled but with longer cells. The carpogonia of this species 

 arise as a branch from the vegetative hyphae and are without special covering 

 hyphae, so frequent a feature in other lichens. The trichogynes bore their 

 way through the compact cortex and rise well above the surface. After they 

 have disappeared — presumably after fertilization — the vegetative hyphae 

 round and between the ascogonia become active and travel upwards slightly 

 converging to a central point. The asci begin to grow out from the asco- 

 genous hyphae of the base before the vertical filaments have quite pierced 

 the cortex. 



Pyrenula nitida has also been studied by Baur'- It is a very common 

 species- on smooth bark, with a thin crustaceous thallus immersed among 

 the outer periderm cells. Unlike most other lichens, it forms carpogonia 

 in spring only, from February to April. A primordial coil of hyphae lies at 

 the base of the gonidial layer, and, before there is any appearance of carpo- 

 gonia, a thick strand of hyphae is seen to be directed upwards, so that a 

 definite form and direction is given to the perithecium at a very early stage. 

 The ascogonial cells which are differentiated are extremely small, and, like 

 those of all other species examined, are uninucleate. There are five to ten 

 carpogonia in each primordium ; the trichogynes grow up through the hyphal 

 strand and, emerge S-iO/x above the surface. After their disappearance, a 

 weft of ascogenous tissue is formed at the base, and, at the same time, the 

 surrounding vegetative tissue takes part in the building up of a plqcten- 

 chymatous wall of minute dark-coloured cells. Further development is 

 rapid and occupies probably only a few weeks. 



In many of the pyrenocarpous lichens — Verrucariae and others — the 

 walls of the paraphyses dissolve in mucilage as the spores become mature, 

 a character associated with spore ejection and dispersal. In some genera 

 and species, as in Pyrenula, they remain intact. 



D. Apogamous Reproduction 



Though fertilization by an externally produced male nucleus has not 

 been definitely proved there is probability that, in some instances, the fruit 

 may be the product of sexual fusion. There are however a number of genera 

 and species in which the development is apogamous so far as any external 

 copulation is possible and the sporiferous tissue seems to be a purely vege- 

 tative product up to the stage of ascus formation. 



In Phlyctis agelaea Krabbe" found abundant apothecia developing nor- 

 mally and not accompanied by spermogonia; in Phialopsis rubra studied 



1 Baur 1901. 2 Krabbe 1882. 



