LICHEN GONIDIA 43 



extract carbohydrates from many sources, so algae can secure their carbon 

 supply in a variety of ways. He affirms that the metabolic activity of the 

 alga in these cultural conditions is entirely normal, and the various cell- 

 contents are formed as in the light. Whether, in this case, starch is formed 

 directly from the acids or through a series of combinations has not been 

 determined. Uhlir', with electric lighting, made successful cultures of 

 Nostoc isolated from Collemaceae on silicic acid, proving thereby that these 

 gonidia do not require a rich nutriment. A certain definite humidity was 

 however essential, and bacteria were never eliminated as they are associated 

 with the gelatinous membranes of Nostocaceae. 



e. Nutrition within the Symbiotic Plant. Culture experiments 

 bearing more directly on the nutrition of lichens as a whole were carried 

 out by F. Tobler^. He proved that the gonidia had undoubtedly drawn on 

 the calcium oxalate secreted by the hyphae for their supply of carbon. In 

 a culture medium of poplar-bark gelatine he grew hyphae of Xanthoria 

 parietina, and noted an abundant deposit of oxalate crystals on their cell- 

 walls. A piece of the lichen thallus including both symbionts and grown on 

 a similar medium formed no crystals, and microscopic examination showed 

 that crystals were likewise absent from the hyphae of the thallus that had 

 grown normally on the tree, the inference being that the gonidia used them up 

 as quickly as they were deposited. It must be remembered in this connection, 

 however, that Zopf ' has stated that where lichen acids are freely formed 

 as, for instance, in Xanthoria parietina, there is always less formation and 

 deposit of calcium oxalate crystals, which may partly account for their 

 absence in the normal thallus so rich in parietin. 



Tobler next introduced lichen gonidia into a culture medium in which 

 the isolated hyphal constituent of a thallus had been previously cultivated, 

 and placed the culture in the dark. In these circumstances he found that 

 the gonidia were able to thrive but formed no colour: they were obtaining 

 their carbohydrates, he decided, not from photosynthesis, but from the 

 excretory products such as calcium oxalate that had been deposited in the 

 culture medium by the lichen hyphae. We may conclude with more or less 

 certainty that the loss of carbohydrates, due to the partial deprivation of 

 light and air suffered by the alga owing to its position in the lichen thallus, 

 is more than compensated by a physiological symbiosis with the fungus*. 

 It has indeed been proved that in the absence of free carbon-dioxide, algae 

 may utilize the half-bound CO2 of carbonates, chiefly those of calcium and 

 magnesium, dissolved in water. 



/ Affinities of Lichen Gonidia. Chodat' has, in recent years, 

 made cultures of lichen gonidia with a view to discovering their relation to 



1 Uhlir 1915. '^ Tobler 1911. ^ Zopf 1907. * Chambers 191 2. ^ Chodat 1913. 



