44 CONSTITUENTS OF THE LICHEN THALLUS 



free-living algae and to testing at the same time their source of carbon 

 supply. He has come to the conclusion that lichen gonidia are probably in 

 no instance the normal Protococcus viridis: they differ from that alga in the 

 possession of a pyrenoid and in their reproduction by zoospores when free. 



Careful cultures were made of different Cladonia gonidia which were 

 morphologically indistinguishable, and which varied in size from lo to i6ju, 

 in diameter, though smaller ones were always present. He recognized them 

 to be species of Cystococais: they have a pyrenoid^ in the centre and a 

 disc-like chromatophore more or less starred at the edge. These gonidia 

 grew well on agar, still better on agar-glucose, but best of all with an 

 addition of peptone to the culture. There was invariably at first a slight 

 difference in form and colour in the mass between the gonidia of one 

 species and those of another, but as growth continued they became alike. 



In testing for carbon supply, he found that gonidia grew slowly without 

 sugar (glucose), and that, as sources of carbon, organic acids could not 

 entirely replace glucose though, in the dark, the gonidia used them to some 

 extent; the colony supplied with potassium nitrate, and grown in the dark, 

 had reached a diameter of only 2 mm. in three months. With glucose, it 

 measured 5 mm. in three weeks, while in three months it formed large 

 culture patches. 



A further experiment was made to test their absorption of peptones by 

 artificial cultures carried out both in the light and the dark. The gonidia 

 grew poorly in all combinations of organic nitrogen compounds. When 

 combined with glucose, growth was at once more vigorous though only half 

 as much in the dark as in the light, the difference in this respect being 

 especially noticeable in the gonidia from Cladonia pyxidata. He concludes 

 that as gonidia in these cultures are saprophytic, so in the lichen thallus 

 also they are probably more or less saprophytic, obtaining not only their 

 nitrogen in organic form but also, when possible, their carbon material as 

 glucose or galactose from the hyphal symbiont which in turn is saprophytic 

 on humus, etc. 



B. Nutrition of Fungi 



Fungi being without chlorophyll are always indebted to other organisms 

 for their supply of carbohydrates. There has never therefore been any 

 question as to the advantage accruing to the hyphal constituent in the 

 composite thallus. The gonidia, as various workers have proved, have also 

 a marked preference for organized nourishment, and, in addition, they obtain 

 carbon by photosynthesis. Chodat* considers that probably they are thus 

 able to assimilate carbon-dioxide in excess, a distinct advantage to the 

 hyphae. In some instances the living gonidium is invaded and the contents 



1 See note Paulson and Hastings, p. 28. 2 Chodat 1913. 



