38 CONSTITUENTS OF THE LICHEN THALLUS 



well as of colourless gonidia filled with the hyphal network. This description 

 hardly tallies with the usual healthy appearance of the gonidial zone in the 

 normal thallus, and it has been suggested that where the fungus filled the 

 algal cell, it was as a saprophyte preying on dead material. 



The gradual perishing of algal cells in time by natural decay and their 

 subsequent absorption by the fungus is undisputed. It is open to question 

 whether the varying results recorded by these workers have any further 

 significance. 



These observations of Elenkin and Danilov have been proved to be 

 erroneous by Paulson and Somerville Hastings ^ They examined the thalli 

 of several lichens {Xanthoria parietina, Cladonia sp., etc.) collected in early 

 spring when vegetative growth in these plants was found to be at its highest 

 activity. They found an abundant increase of gonidia within the thallus, 

 which they regarded as sporulation of the algae, and the most careful methods 

 of staining failed to reveal any case of penetration of the gonidia by the 

 hyphae. 



Nienburg^ has published some recent observations on the association of 

 the symbionts. In the wide cortex of a Pertusaria he found not only the 

 densely compact hyphae, but also isolated gonidia. In front of these latter 

 there was a small hollow cavity and, behind, parallel hyphae rich in contents. 

 These gonidia had originated from the normal gonidial zone. They were 

 moved upward by special hyphae called by Nienburg "push-hyphae." After 

 their transportation, the algae at once divide and the products of division 

 pass to a resting stage and become the centre of a new thalline growth. A 

 somewhat similar process was noted towards the apex of Evernia furfur acea. 

 Radial hyphae pushed up the cortex, leaving a hollow space over the gonidial 

 zone. Into the space isolated algae were thrust by "push-hyphae." In this 

 lichen he also observed the penetration of the algal cell by haustoria of the 

 fungus. He considers that the alga reaps advantage but also suffers harm, 

 and he proposes the term helotism to express the relationship. 



An instructive case of the true parasitism of a fungus on an alga has been 

 described by Zukal^ in the case of Endomyces scytonemata which he calls 

 a "half-lichen." The mature fungus formed small swellings on the filaments 

 of the Scytonema and, when examined, the hyphae were seen to have attacked 

 the alga, penetrating the outer gelatinous sheath and then using up the 

 contents of the green cells. It is only after the alga has been destroyed and 

 absorbed, that asci are formed by the fungus. Zukal contrasts the develop- 

 ment of this fungus with the symbiotic growth of the two constituents in 

 Ephebe where both grow together for an indefinite time. 



Mere associated growth however even between a fungus and an alga 

 does not constitute a lichen. An instance of .such growth is described by 



1 Paulson and Hastings 1920. ^ Nienburg 19 17. * Zukal 1891. 



